Frictional Electricity

FRICTIONAL ELECTRICITY BY HERMANN F. VIEWEG

Introductory It was known to the ancients that substances like glass or amber acquire the peculiar property of attracting small pieces of other matter when rubbed with cloth or fur. The attraction between the loadstone and iron was also known. It was not realized that these phenomena were distinct, and the earlier writers did not differen tiate between them. Gilbert’, in his interesting treatise on the magnet, pointed out that these attractions were of two kinds. The first was that between iron and the loadstone; this he called magnetic. The other was the attraction exerted by amber on small bodies. Gilbert discovered that a number of other substances could acquire this same property of attraction, and he devised a simple electrometer to study their action. Those bodies which like amber ( ~ ~ X E K T ~ O V ) would attract others when rubbed he called “electrics” ; others which lacked this property he called ‘Lan-electrics”. Gilbert may thus be said to have started the study of frictional electricity. During the seventeenth and eighteenth centuries, numerous investigations were undertaken; these, however, were directed mainly towards a study of the properties and action of the frictional electricity, developed by rubbing a few standard substances, rather than towards the examination of new substances, or an explanation of the origin of the electricity. The accounts of many of these experiments are interesting, and often very amusing, but they are of little scientific value to us a t present. For a long time, a substance was thought of as being either distinctly positive, or distinctly negative, arid it was believed that the sign of its charge would normally be the same whenever it was rubbed. Wilckez was the first t o bring out the idea that materials could be arranged in a series grading from those with the greatest tendency to become positively charged to those becoming most readily negative. Thus any member of the series might become either positively or negatively charged, depending on the substance with which it was rubbed. Wilcke arranged a few materials in order, thus establishiny the first simple frictional electric series. Various other investigators since then have worked along the same line, adding new substances, and re-arranging those already studied, Among these should be mentioned Faraday,3 who established a series, more complete than the previous ones. He determined the fact that water must be a t the positive end of the series since it charged everything else negatively. Guilielmi Gilberti Colcestrensis: “De Magnete,” Book 11, Chapter 11, London (1600). Wilcke: Phil. Trans., 11, 401 (1759). 3 Faraday: Exp. Res., 2, zo75-z115 (1533).

1 2

866

HERMAKN F. VIEIVEG

As the subject was studied more and more, it was found that arranging such series was not quite as simple as it seemed. Thus, although Riessl believed that every substance had its own definite position in a frictional electric series, he had difficulties in proving it, because of certain contradictory results he encountered. He explained these as being due to abnormal surface conditions, the effect obtained not being characteristic of the material itself. In spite of these troubles, Riess managed to arrange most of the things he studied in a series. His most valuable achievement was the determination of the fact that an equal amount of both kinds of electricity is generated by friction. While some were working to arrange various materials in order, others were more interested in developing the laws governing the generation of frictional electricity. Peclet2made a thorouph study of a few substances, and found that there was a definite maximum surface density of charge which one substance could acquire by friction on a specified second substance. This maximum he found was independent of the velocity of rubbing, the pressure applied, and of the area of surface involved. Riecke3 derived the mathematical expressions showing the relations between the density of charge, the area of surface, and the rate of rubbing. His calculations, as he showed, agree quite satisfactorily with the results of Riess. With all these data of many workers available, it was to be expected that efforts would be made to offer an explanation for the cause of frictional electricity. In 1879, Helmholtz4 proposed the theory that the fundamental cause is a contact difference of potential which exists between any two materials, According to this theory, when two unlike surfaces are broupht together, there is a concentration of positive charge on one surface and of negative on the other. It can be shown theoretically that on separating the two, if either be a non-conductor, the contact potential difference is greatly magnified, and the two surfaces will therefore be charged oppositely and at very high potentials. If both surfaces are conducting, the charges will neutralize each other on separation, at the l%stinstant that the two substances are in contact at any point. Thus two insulators, or an insulator and a conductor, will remain charged after separation; in the case of two conductors no appreciable charge persists. This view was supported by Hoorweg,j who measured the contact potential differences between various materials. He found that friction between two substances charged them the same way as one would expect from their difference of potential while in contact. The existence of a contact difference of potential between metals and insulators, as well as between insulators themselves, was shown experimentally. Hoorweg believed that the charges were due in part to a thermo-electric effect caused by local heating due to friction. Riess: Pogg. Ann., 236, 589 (1877). Peclet: Ann. Chim. Phys., 57, 337 (1834). Riecke: Wied. Ann., 3, 414 (1878). 4Helmholta: W e d . Ann., 7, 337 (1879). 6 Hoorweg: %Tied. Ann., 11, 133 (1880). 1

2

FRICTIONAL ELECTRICITY

867

Further study of contact difference of potential between various materials was made by Christiansen.’ The Helmholtz theory does not predict what the relative polarity of two substances should be. Attempts have been made to determine the relation between other physical properties and the position in a frictional electric series. After investigating quite a number of di-electrics, Coehn2 postulated the following: The order of non-conductors is the same as that of their dielectric constants, the higher di-electric constants corresponding to a greater tendency to become positively charged. In a later investigation it3 was also shown that the difference of potential is proportional to the difference in dielectric constant. On the other hand, Hesehus4 believed it was a question of the relative density of the surfaces, the more dense becoming positively charged, electrons flowing to the less dense, this being aided by a change in the surface tensions a t the points of contact. It has been shown by Melanderj that the temperature of a substance affects radically the sign of charge that it acquires on being rubbed. The few materials investigated by him became more easily positively charged as their temperatures were increased. He proposed the idea of “electronic temperature” as distinct from the ordinary molecular temperature. Two substances a t the same molecular temperature will in general have different electronic temperatures, the substance having the higher giving up its electrons more readily than the other. With an increase in heat content, the electronic temperature will also be raised, and the tendency to acquire a positive charge increased. Thus a reversal in the potential difference might be expected if the ordinarily negative substance were heated sufficiently. Although the study of frictional electricity has not reached the point of being a quantitative science, a few researches of a quantitative nature have recently been carried out. Morris-0wen6 has studied the relation between the frictional work done in rubbing and the magnitude of charge developed, varying the pressure exerted between the two surfaces. It was found that the maximum surface density of charge obtainable was independent of the pressure used in rubbing, but that the greater the pressure applied, the smaller was the work required to attain the maximum charge. Similarly, Morris- Jones’ measured quantitatively the maximum charge of several materials when rubbed with silk, flannel, and chamois leather; also of certain metals rubbed with silk. Assuming that the rate of charging was proportional to the rate of work, and that the charge leaked from a surface Christiansen: Wied. Ann., 53, 401 (1894). Coehn: Wied., 64,217 f1898); 66,1191 (1898). 3 Coehn: Ann. Physik, (4) 30, 777 (1909). 4Hesehus: Physik. Z., 2, 7 5 0 f1901); J. RUSS.Phys. Chem. SOC.,34, I (1902);35, 575 (1903); 37, 29 (1905). Melander: Physik. Z., 8, 700 (1907). 6 Morris-Owen: Phil. M a g , 17,457 (1909). ’,Morris-Jones: Phil. Mag., 29, 261 (1917). 1

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HERMANN F. VIEWEG

a t a rate proportional to its charge, an expression was derived showing how the charge varied with the work done. The maximum charge would be attained when the rate of charging just equalled the rate of leakage. The results obtained agree quite well with the values predicted. The recent work of Shawl is no doubt the most complete and thorough investigation that has yet been made of the generation of frictional electricity. A large number of substances were arranged in a “tribo-electric” series. The surfaces, in addition to being studied under normal conditions, were investigated under abnormal conditions, such as after and during heating; under great pressure; while flexed; and after polishing or grinding. The general effect of each of these abnormalities of surface was determined. Shaw divided all his materials into two groups, “A” and “B”, depending on how they acted under these abnormal conditions. He postulated that there were two oppositely charged layers near the surface of any substance; those in group “A” having the negative layer outside, while in group “B” the positive layer was outside. Abnormal conditions would cause a disarrangement of these layers and might even reverse their positions. No explanation was given to show why any particular substance should belong to the group in which its tribo-electric properties placed it. All that has been done in. this field up to the present may be summarized as follows: Various frictional electric series have been established, and a considerable number of materials have been assigned their positions in them. The relation between position in the series and other physical properties, especially the di-electric constant, has been pointed out. A few quantitative researches have been undertaken. The effects of certain abnormalities of surface have been studied. The existence of contact differences of potential between different solids has been shown. No satisfactory explanation of the causes and origin of frictional electric charges has been given.

A Frictional Electric Series The present investigation was undertaken with the following objects in view: ( I ) to arrange a frictional electric series of suitable materials, ( 2 ) to explain, if possible, from such a series how various factors determine the position a substance takes, (3) to learn what the effect of moisture is, and to what extent this may account for anomalies previously observed. The first step, then, was to study a group of materials, chosen as adapted to the purpose, with reference to the charges produced when different surfaces are rubbed together; and from’the results to arrange a frictional electric series. In selecting the materials to be used, it seemed best to use crystalline substances of definite composition, and to observe the effect of definite faces. Many of the materials usually employed by other investigators are of such Shaw: Proc. Roy. SOC.,94A, 16 (1917).


869

indefinite nature that it does not seem probable that results obtained with them can be interpreted; nor are they always reproducible. Such substances as furs, while they serve excellently in the generation of frictional electricity, are surely of quite variable properties. It does not seem reasonable to require all cats to have furs with identical electric characteristics. In fact, Shaw found in his experiments that one fur might vary from one part to another, being positive a t one point and negative at another, with reference to the same substance. Nor is one impressed with the dielectric constant of bacon as being an accurately defined physical property. Similar objections apply to such indefinite substances as “filter-paper,” “blotting-paper”, “sealing-wax”, “wood”, et cetera. In using such things, one is merely adding unknown factors, without which the problem is already sufficiently complicated. On the other hand, we are dealing with something absolutely definite when we use a specified face of a specified crystalline substance. While, of course, not all the crystalline structures of the materials studied are known, they nevertheless do have definite structures, the properties of specific faces are definite, and results can be duplicated. In many cases, t8hearrangements of the component atoms have been determined, and we know just how each face is constructed. Also, sufficient work has been done on the distribution of electricity in atoms, to give a good idea of how they are made up. The attitude adopted in this part of the investigation has been that the ultimate solution of the problem lies in a consideration of the electrical structure of the surfaces in question, and that the bases for such a consideration are, first, the arrangement of the component atoms, for which one depends chiefly on the results of X-ray analysis,1 and, secondly, the structure and electrical properties of the atoms themselves, as indicated by the theoretical and experimental results of the Bohr and Lewis theories.2 In accordance with this view, various crystals were used, these being mainly well-crystallized minerals, the choice of those to be used depending to some extent on what was readily available. In addition, a few of the standard materials were used, in order to correlate this series with others previously established. In the experimental work itself, care was exercised to avoid as much as possible all effects that might cause anomalous and inconsistent results. In particular, attention was paid to having the surfaces clean and dry. Fresh surfaces were broken where possible, as with minerals having good cleavage, others were scraped, and metals, were cleaned either chemically or mechanically. All substances were dried over sulfuric acid before testing. The specimen to be investigated was fastened with sealing-wax or glue to a glass rod about I O cm. long. This served as an insulator and also allowed materials to be handled at sufficient distance to prevent loss of charge by induction. The other end of the rod was equipped with a wire b o k , so that it might be hung up in a desiccator. See W. H. and W. L. Bragg: “X-Rays and Crystal Structure” (1924). See Andrade: “The Structure of the Atom” (1924).

870

HERMANN F. VIEWEG

In rubbing two surfaces together, a pressure was applied, and the rubbing done rather vigorously. It was found that successive results might be inconsistent unless this was done. This is'no doubt due to the adsorbed films which exist on some surfaces, the properties of which, rather than those of the surfaces themselves, are tested by gentle rubbing. Such films may be removed or penetrated by proper application of force. To measure the charges so obtained, use was made of a sensitive electrometer, of the Dolezalek quadrant type. One set of quadrants was grounded, the other connected to a simple condenser a t about two metres distance. This consisted of two circular copper plates of 3 cm. radius, separated by a glass plate, 3 mm. thick. The needle of the electrometer could be charged to a negative potential of j j volts, from the negative terminal of a direct-current line. A U-tube of water was put in series with the instrument, this hiph resistance protecting it in case of short-circuit inside the electrometer, and still allowing the potential difference of 5 j volts to be maintained. All wiring from the electrometer was thoroughly insulated, a grounded copper tube enclosing the rubber tubing in which the wiring was contained. The instrument itself was surrounded by a grounded metal container. These grounded conducting shields were a protection from outside electrostatic effects. The image of a lamp, reflected from the mirror of the electrometer, was focussed by a lens on a scale, about three metres distant. No attempt was made at quantitative measurements, although it was desired to have some idea of the magnitude of the charges. When the upper plate was charged to a potential of 15 volts, the electrometer deflection was 2 0 scale divisions. The capacity of the two plate condenser can be calculated to be . O O O O ~microfarads. A charge of 4X 10-l~coulombs, or 2 . j X IO-^ electrons, was therefore equivalent to one unit of the scale. On this basis, the charges were found to vary between 10-9 and 10-l~coulombs, and in each case equal and opposite on the two substances. Using the method just described, the following series was established, each substance being charged positively on rubbing with any one below it, and negatively by any above it. The composition, crystallization, and crystal face used, is stated for each member of the series, except that where no crystal face is mentioned, massive or non-crystalline specimens were used.

Discussion Whenever two unlike surfaces are brought into contact, there is a tendency for electrons to pass from one surface to the other. As a result, that surface which possesses the greater attraction for electrons or the lesser power to emit them becomes charged negatively, while the other acquires an equal positive charge. We may think of each substance as having a characteristic electron pressure: or, as Melander calls it, an electron temperature. That two metals have a definite contact potential difference, a function of the temperature, is well-known from the Peltier effect. I n the case of two non-conductors or of a non-conductor and a conductor a contact potential


TABLE I

Substance Serpentine Asbestos Fur Topaz

The Frictional Electric Series Positive End of Series Composition Crystallization, Monocl. HiMg3Si209 Silicate Organic Rhombic (AIOH)kX04

11

Mica Glass Calcite

11

KH2AldSi04) 3 Silicate CaC03 11

11

Aragonite Cheesecloth Barite

17

Organic Bas04

11

Monocl. Hex. 17

Rhombic Rhombic

11

11

11

71

11

11

Quartz 11

Magnesiiim Fluorite Lend Fluorite Gypsum 11

Celestite Zinc Orthoclase Anhydrite Beryl Blotting paper Sealing-wax Sphalerite Magnetite Galena Pyrite AIolybdenite Copper Antimony Stibnite Silver Silicon Sulfur Rubber

Si02 11

Hex. 11

J4g CaF2 Pb CaF2 CaS042H20 11

SrS04 Zn KAISi30e CaSOl ReRAI2 Si6OI8 Organic

Isometric 11 11

Monocl. 17

Rhombic Isometric Monocl. Rhombic Hex.

11

Zn S Isometric 17 Fe8O4 77 PbS 11 FeS2 3lOSZ Hex. cu Isometric Sb Hexagonal SbsSa Rhombic Ag Si S Rhombic Organic Negative End of Series

872

HERMANN F. VIEWEG

difference has been found by Helmholtz, by Hoorweg, and by Christiansen. Melander’s work indicates that this, too, varies with the temperature. The existence of such a difference of potential is disputed by Morris-Owen, because he could obtain no frictional charge from mere contact of two dielectrics. The real reason for this is that mere touching does not bring any appreciable area of either substance into contact with the other. Even rubbing does not bring a large percentage of the total area into effective contact. This is indicated by the fact that when a crystal is pulled apart on a cleavage plane where opposite faces are unlike, the charges so developed are very much larger than those obtained by rubbing the same surfaces together. One cannot hope by rubbing the smoothest surfaces to bring a large portion of the total area into real contact. It is therefore reasonable to expect that mere touching has no great effect. On the other hand, the fact that the separation of two surfaces in actual contact at molecular distances, without any rubbing, causes such great charges, is strong evidence in favor of a contact potential difference, Shaw has expressed the opinion that, if the charge is due to a transfer of electrons, the metals with all their “free” electrons ought to be at the positive end of the series. Since they are not, he doubts the explanation given here. There seems to be no basis for such an objection. The electrons of a metal may be “free” in the Pense that they can readily move through the material when the proper potential difference is applied, but there is no excess of electrons ready to be given up, when a metal is at zero potential. Hence a metallic element will take its place in the series according to its electronic structure, and not at a special place due to its metallic nature. Let us consider what happens when two unlike surfaces are in contact. Electrons from each substance will, in their rotations or oscillations, come within the sphere of activity of some atom of the other substance. Occasionally one of the electrons, being more strongly attracted by a positive nucleus with which it was not originally associated, will stay with it. There will be a mutual exchange of electrons; but that surface which loses electrons less readily will get more than it gives up. On separation it will have acquired a negative charge, the other a positive charge. The direction in which the effective transfer of electrons takes place is dependent on the nature and structure of the atoms in each surface. Each atom is a definite aggregate of electricity; the atoms are arranged in a definite manner in crystalline matter, but in a random way in amorphous matter. Experimentally it is found that the nature of the atoms, that is, the chemical composition of the surface, is a more important factor than the arrangement of the atoms. This is shown by the fact that, although different faces of the same crystal have different positions in the series, the general position of a chemical compound is independent of the face used. In the case of CaC03, the two crystalline forms, calcite and aragonite, occupy adjacent positions. Theoretically one might find something intermediate between two faces of any crystal; however, in this work the only case of this kind observed was that


8 73

of lead, intermediate between fluorite (111) and (100). Jt is interesting to note, too, that in case massive material is used, its position is intermediate between the positions of the faces of the crystal. The differences between two compounds are greater than the differences between two faces of the same compound; but that the latter differences exist is certain. In no case was it possible to obtain any charge from two like faces of a crystal. When we come to a consideration of any particular case, the problem becomes more difficult. The study of the arrangement of atoms in crystals, and that of the arrangement and motions of the electrons in these atoms, have not advanced far enough to be entirely sufficient. However, with the information a t present available, it is possible to explain some of the less complicated cases. The most simple substances to consider are the elements. Every element has a definite ionization potential, that is, the amount of energy required to remove an electron from an atom is characteristic of the element. This can be calculated on the basis of its structure under the Bohr theory. I n many cases this has been determined experimentally and found to agree closely with theory. There are other phenomena such as thermionic effect, and photoelectric effect which may be used as a basis for determining the “electron affinity”’ of an element. The relative order is the same by these different methods. Taking the data that are available,’ we find the following relations for the elements in our frictional electric series: Element

Ion. Potential.

Electron Affinity

Ag cu

Zll Pb Mg

9.5 8.0 7.8

4.1 4.0 3.4 ,

2.7

The order of the elements in the frictional electric series is thus seen to correspond to the affinity for electrons indicated by other phenomena. The so-called normal electrode potentials are also a measure of the relative tendencies of metals to lose electrons. For example, the electrode potential of Mgis more negative than that of Ag. This means that the reaction takes place more readily than

Mg -+ Mg+++2 0 Ag +Ag++ 0

and that therefore a iclg electrode will give up electrons at a greater pressure than will a Ag electrode. It does not mean, as it might seem at first, that Mg has a greater attraction for electrons than Bg has. Those atoms which actually react in each case acquire a positive charge, this occurring more easily in the case of 14g. The fact that the Mg metal which has not reacted has a more negative potential than the inert mass of the silver, is due to the greater tendency of the Mg atom to lose electrons, the reaction of course taking place ~~

Smithfionian Physical Tables,” 404 (1920).

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HERMANN F. VIEWEG

continuously only when the excess electrons are removed from the electrode. The electrode potentials are therefore in accord with other phenomena depending on electron affinity. Since the conditions under which relative electron affinities are determined differ so greatly in the cases of measuring normal electrode potentials, and of arranging a frictional electric series, one should not expect that the order of a set of elements would be exactly the same in both cases, However, the agreement between the two methods is good. In considering compounds we are dealing with atoms of more than one kind. The ease with which a surface gives up its electrons will depend on ho\a the atoms are arranged with reference to that surface. Therefore different faces of a crystal will have differences of potential. Several examples of this fact are found in the series. It will be interesting to consider a substance of which the crystal structure has been determined, such as fluorite,l CaF2. In the (100)face the outer two layers of atoms are arranged like this: Surface

F

F CR

In the

(I I I)

F Ca

F Ca

F Ca

F Ca

F Ca

face the arrangement has been iouncl to be:

F

F

F

F

Ca

Ca

Ca

Ca

Surface F F Ca

E Ca

’ c‘a

P

E Ca

’ Ca

The distance between the layers is 1.73 times as great in the second case as in the first.2 Since in the octahedron face the calcium layer is further from the surface, and also since the atoms are less exposed than in the cube face, it is to be expected that the properties of the Ca atom will be relatively more important in the (100)face than in (TII). In considering the properties of atoms in a crystal it must be remembered that the electrical structure of an atom is somewhat modified when it is part of a compound. That is, in C‘aIT2the li‘ atoms have each acquired one electron from some Ca atom, or a t least are holding one in common with a Ca atom. Under such an arrangement the fluorine atom has no further tendency to take an electron from another atom; the Ca atom, having two of its electrons held by E’ atoms, has some tendency to take an electron if it can get one. Therefore that face, (IOO), in which the properties of Ca are relatively more predominant, should acqiiire the negative charge with reference to the other face. Experimentally, the octahedron is found to be positive with reference to the cube, as predicted. Bragg: “X-rays and Crystal St,ructure”. 1.36 Xro-* cm., and 2.35 XIO-8 cm., respectively.


875

Another interesting case to consider is that of a crystal in which two faces perpendicular to the same line are different. Thus in topaz. where other evidence has shown that 001 is different from OOT, it is found that the former charges the latter negatively. This is true either when the crystal is broken apart on its cleavage, without rubbing, or when the two faces are rubbed together, Probably when the internal structure of topaz is determined, it will be found that those atoms which more readily lose electrons are on the outside of the 001 surface, thus putting the atoms with greater attraction for electrons on the outside of OOT. With two substances of different composition, the problem i,4 more comples. If they are salts of different metals with the same acid radical, the differences are essentially those between the basic atoms themselves. Thus barite, BaS04, charges celestite, SrSO4, negatively, and this charges anhydrite, CaS04, negatively. This means that the tendency to lose an electron is, greatest for Ba, and least for Ca, and is, of course, in agreement with the fact that Ca is nearer the noble elements in the electrochemical series than Ba is In case both the acid and base are different, it may be difficult to foresee how much the effect of each will count. In general the position of a compound will be a resultant of the electron affinities of the different atoms, but the determination of the relative importance of each atom may be complex. A thorough study of this subject would involve a consideration of various series of similar compounds, in order to determine the effects of different atoms and radicals. Such a study has not been attempted in this investigation. However, enough has been brought out here to show that this general method of attach leads to results in accord with facts familiar to us from other phenomena. No doubt as knowledge of the electrical structure of matter increases, further consideration from this same point of view will throw more light on the subject of frictional electricity.

“Coehn’s Rule” Since Coehn’s rule is perhaps the most valuable generalization of frictional electric effects that has yet been presented, it will be considered here. It states that a frictional electric series of non-conductors from positive to negative corresponds to a series from highest to lowest dielectric constant. This offers no explanation as to why the two series have the same order, why a high dielectric constant should make a substance more positive, or why good conductors occur in a frictional electric series between dielectrics. Perhaps from a consideration of the electronic theories here discussed, we can see what this rule means. The dielectric constant of a medium is a measure of the force existing between two opposite charges in the medium. The greater the dielectric constant the less will be the force. Without attacking the problem quantitatively, we can see that, in the higher dielectric medium, an atom which is positively charged because it has lost an electron will exert a lom7er attractive force on

876

HERMANN F. VIEWEG

an electron a t the surface than will a positively charged atom in the other medium, when two non-conductors are in contact. The resultant effect will be to give the medium of lower dielectric constant a better chance to regain any electrons it may have lost, and it is therefore to be expected that it would be negatively charged. This is in agreement with the rule. To refer the sign of charge back to the dielectric constant is really not explaining anything. Let us consider what this constant actually represents. The arrangement of the electrical components of any medium determines its electrical properties, one of which is the dielectric constant. To say that one medium has a higher dielectric constant than another means that in the first case positive and negative charges are so arranged in its structure that an electron is attracted by a unit positive charge with a smaller force than it would be under similar conditions in the second case. The dielectric constant is merely one physical property which measures the relative attraction between an electron and the positively charged mass of the substance. I n the case of conductors the methods usually applied in the determination of dielectric constants fail. The reason for this is that, since opposite charges do not persist for any appreciable time in a conducting medium, the force between such charges cannot be directly measured, Conductors, however, occur in the frictional electric series, in positions determined by their relative attraction for electrons, independent of their conducting properties.

Applications A knowledge of the way different faces of the same crystal charge each other may be a means of giving us certain information which is more difficult to obtain by other methods. A point brought out by this work is the possibility of using a frictional electric study to determine whether or not two faces of a crystal are alike. This method is apparently very sensitive. For example, the two faces of gypsum parallel to the good cleavage are usually considered to be alike. However, the fact that 010 and O i O charge each other oppositely, either when a gypsum crystal is pulled apart on its cleavage plane or when the two faces are rubbed together, indicates that the two faces are different. The Effect of Moisture So far we have considered only ideal cases, that is, the surfaces were assumed to be clean and perfectly dry. Under ordinary conditions this is usually not true. Films of adsorbed moisture and other impurities may be present. It is well known that on damp days it is difficult to generate frictional electricity. Results so obtained are often inconsistent. Many previous investigators have had peculiar results because of this. Some were not aware of the fact that they were having any trouble; others, knowing it, cheerfully ignored it; while a few, Hoorweg for example, tried to work in the absence of moisture when it became obvious that inconsistencies were due to it. No one, up to the present, has made a systematic study of the effect of moisture, although such a study is evidently of great importance.


.

877

As a preliminary experiment, the materials previously arranged in the series were moistened by allowing steam to condense on them. On rubbing any two together, it was found that both would acquire a positive charge. This however, was small compared with the charges developed on the dry substances. Such a method is obviously crude, and it was therefore decided to make a more careful and systematic study of a few substances. Those selected were : quartz; glass; composition insulation (CI) ; sealing-wax; and silver. This is a representative set, distributed throughout the series. Five large desiccators were arranged with glass stands so that these samples could be suspended in them. The solutions in the desiccators were: (I) HZS04; ( 2 ) 56% H2S04, 44% H2O; (3) 43% H2S04, 57% H 2 0 ; (4) 30% H2S04, 70% H20; ( 5 ) HzO. In the temperature range 15' to 2 5 O C . these solutions will maintain aqeous vapor pressures above them in the ratio of o%, 25%, 5 0 % ~ 7 5 7 c , and 100%. The relative humidities of the atmospheres in these desiccators therefore had the same relations to each other as the vapor pressures. Specimens were placed in a desiccator and allowed to remain there for at least 24 hours to reach equilibrium, They were then rubbed on one another, and the charge measured with the electrometer. After discharging the specimen, by holding it in the ionized region near a gas flame, it was returned to the desiccator to again come to equilibrium before another determination. In this manner every possible pair of the five substances was tested at each humidity. The results brought out the interesting fact that in every case the effect of water was the same. The exact value of the charge varied with the conditions of rubbing, but the effect of the moisture was always to add a positive charge to each surface. The naturally negative substance might be negative, neutral, or positive; the other surface was always positive. In the extreme case where the substances had come to equilibrium with an atmosphere saturated with water vapor, the general result was to charge any two substances positively.' However, the two charges were not equal; the substance which would normally be negative having the lesser charge. The difference in magnitude between the two charges was quite distinct. This indicates that one effect of moisture is to add to each surface a positive charge, superimposed upon the natural tendency to charge one positively and the other negatively. There are three factors, affecting the sign of charge, to be considered. ( I ) The first is the one just mentioned. Its explanation mill be discussed later. ( 2 ) The contact of one solid against the water on the other mill charge the solid negatively and the water positively. Similarly the second solid acquires a negative charge from the water on the other surface. These effects may be small but the result will be to leave an excess of positive charge on that As will be shown later, the rorresponding negative charges, which must have been generated, were acquired by the air.

878

HERMANN F. VIEWEG

surface which under normal conditions would charge positively and an equal negative charge on the other. (3) Whenever, by breaking through the moisture film, the two solids come into direct contact, they will charge according to their natural tendency, one positively, the other equally negatively. The combined effect of ( 2 ) and (3) is to charge the two solids equally and oppositely. ( I ) adds a positive charge to both materials. The resultant will depend on the relative magnitudes of the two effects. (I) will be more predominant at higher humidities; the combined effect of ( 2 ) and (3) will increase as the two surfaces are further apart in the series. Thus a t 100% humidity the added positive effect was sufficient in all cases to overcome the other effects, and both substances were positively charged, Of course, the naturally positive substance had the greater charge. With a relative humidity of 7;70the effect due to moisture was not so great. In some cases instead of both substances being positively charged, one acquired a positive and the other a much smaller negative charge. Thus quartz and silver are near enough in the series so that the positive charge due to moisture was greater than the negative charge acquired by silver from contact with quartz; both were therefore positively charged. Similarly the CI is near enough to quartz so that both acquired a positive charge; but the difference between silver and CI is great enough to make the former charge slightly negatively. At 5076 humidity the effect due to moisture had decreased considerably. Under these conditions, the water was sufficiently important only in the cases of sealing-wax and silver, and of CI and quartz to cause both to charge positively; other pairs of substances charged unequally and oppositely. The importance of moisture at 2 5 Y c humidity was relatively small. KO two substances acquired a positive charge simultaneously. The negative charges were more nearly equal to the positive charges than a t higher humidities. This indicates that the natural opposite charges are greater than the positive charge added to both surfaces due to the water a t this humidity. It can readily be seen from these data that they support the previous statement: The effect of moisture is an added positive charge,l increasing with the humidity and decreasing as the distance in the series between substances increases. If the effect becomes greater with the amount of water present on a surface, as would happen with increasing humidity, there must be some pairs of materials which differ so greatly in the amount of water they will adsorb a t the same humidity that there will be a marked difference in the magnitude of the moisture effect. To verify this the following pairs were selected: serpentine and fur; calcite and cheesecloth; and beryl and blotting-paper. In each case the first is normally positive to the second; they are very near each other in the series; and the second is capable of much greater adsorption of moisture. These substances were allowed to come to equilibrium with water vapor and then rubbed on each other. In each case it was found that the The corresponding negative charge is acquired by the air in contact with the film.


879

normally negative substance of each pair, could, due to its much greater adsorption of moisture, become more positive than the other, normally positive, material. Since, at this point, it seemed evident that the effect of moisture must be due to the same phenomena as Lenard’s “WasserfallelektrizitSit”,l it was thought desirable to see whether solutions of electrolytes behaved similarly. It was found that if two solids were rubbed together with a 2% NaCl solution on their surfaces the previous effect was reversed, that is, a negative charge was added to each. This would lead one to expect that a t some lower concentration there must be a point where the solution would not add a charge of either sign, On trying various strengths of solution with a few pairs of solids,2 it was found that below .OI N the added charge was in the same sense as for water, while above , I N it was the reverse. This method was not sufficiently sensitive to determine the effects between these strengths but it fixed the approximate limits within which the inversion point must lie. A . I S solution of HC1 was also tried with these materialsj3 its presence added a negative charge to both solids. No appreciable charges were observed on two surfaces covered with . I r\’ NaOH, when they were rubbed together. At normal concentration, NaOH solutions acted oppositely from water, adding a negative charge to each solid. The facts, then, as to the effects of the presence of films of moisture, or of solutions of acid, alkali, or salts are quite simple. Water tends to add a positive charge; the others, when sufficiently concentrated, add a negative charge. The real problem, therefore, is to explain the causes of these effects. In the case of pure water, the excess of positive charge on the substances being studied shows that negative electricity has disappeared, presumably into the air. Similarly, the solutions must have lost a positive charge to the air. This indicates that we must be dealing with another variation of a wellknown phenomenon, the “Wasserfallelektrizitat” of Lenard. During the rubbing together of two solids covered with liquid films, the result is the mechanical disintegration of the films, causing the liquid to be charged electrically, the air in contact with the film acquiring an equal opposite charge. A study of the causes of this effect is the same as an investigation of the reasons for various similar phenomena. Before discussing any further experimental work, it would be well to see what these other effects are, and what the most important researches along this line have been.

“Wasserfallelektrizitat” Although a few previous observers had noted the fact that the air near water-falls is negatively charged, it was Lenard4 who first made a thorough This will be discussed on p. 880. different combinations of: topaz, glass, calcite, quartz, orthoclase, sphalerite, and silver. 3 Calcite and sphalerite were not used because the acid would attack them. Lenard: Wied. Ann., 46, 584 ( 1 8 9 2 ) .

* The

880

HERMANN F. VIEWEG L

study of the subject, His original paper may be said to be the basis for all further work in this field. Lenard showed that when water drops were broken up in a falls, the air was charged negatively, and the water positively. He also found that the sign of charge was the same when different gases were used. Various liquids and solutions were similarly tested, I n testing NaCl solutions of different strengths, the charge of the liquid was observed to be positive up to .OII% NaC1, and negative above this concentration. Lenard believed the cause of the phenomenon was a contact potential difference between liquid and gas. The inverse, namely the bubbling of gas through liquids was first studied by Kelvin, MacLean. and Ga1t.l Although some of their results were incorrect, they established the fact that a gas would become charged when bubbled through a liquid, such as water, or a solution of an electrolyte. Kosters2 determined the charges on hydrogen and oxygen when bubbled through various liquids. In either case, the gas acquired a negative charge from water, whereas electrolyte solutions above a certain concentration charged the gas positively. By far the most thorough work on this phase of the subject was done by R40zer.~ I n particular, hydrogen and oxygen were bubbled through quite a number of liquids and solutions of different concentrations. Qualitatively the effect of the two gases was the same. Electrolytes below a certain strength charged the gas negatively and above this the gas became positively charged. This concentration was determined for various acids, salts, and bases. It was found to be lower for acids than for their salts, while that for the corresponding base was higher. Mozer suggested that the action of an electrolyte might be due to adsorption of its ions by the gas, Another interesting point that he brought out was the fact that a bubble of gas in a liquid is electrically. charged, and with the same sign that it bears on leaving the liquid. This was shown by the motion of the bubble under an electric field. This particular point has been the subject of several researches by McTaggart4 who showed that an air bubble in contact with water bears a negative charge. The charges on bubbles in various solutions were tested. With Th(K03)4in solution the charge on the bubble could be reversed. Numerous other workers have studied this subject. The essential facts have all been in agreement with those brought out in the papers just mentioned. The results are the same whether liquids be broken up in a gas atmosphere, or a gas be bubbled through a liquid, or even when a gas bubble is in contact with a liquid. With water the gas charges negatively, and with electrolytes above a certain strength positively; the liquid always bearing an opposite charge. Kelvin, Maclean, and Galt: Proc. Roy. Soc., 57, 335 (1895). Kosters: Wied. Bnn., 69, 12 (1899). Mozer: Dissertation, Gottingen (1913); Coehn and Mozer: Ann. Physik, (4) 43, I048 ('914). McTaggart: Phil. Mag., 27, 297 (1914); 28, 367 (1914); 44, 386 (1922).


881

An interesting summary of all the previous work in this field is found in an article by Becker.l Lenard2 also has summarized what was done up to 1915. According to his latest theory, the cause of electrification is not a contact potential difference, but is caused by mechanical processes, and is due to the liquid only. It is assumed that in water there exists a double layer of ions a t the liquid-gas interface, the OH- ions in the outer, and the H+ ions in the inner layer. Electrolytes tend to have their cations on the outside, and their anions nearer the interior. Mechanical breaking up of a drop would tear off the outer layer most readily, and the gas would acquire a charge due to the fine particles of charged liquid in it. The greater the concentration of an electrolyte, the more positive would be the outer layer of the liquid. This explanation accounts for the charges developed by bubbling gases through liquids, or by pulverizing liquids in a gas. It does not account for the charge on bubbles, which has been definitely observed by McTaggart and others. With this brief summary of previous work in mind, we can proceed to a consideration of the causes underlying all these similar phenomena. In the cases of bubbling, or of water-falls, it is a question of the results of disturbing conditions suddenly at a liquid-gas boundary, With bubbles in liquid, the conditions existing on the surface before anything has disturbed them are evident. The potential difference between gas and liquid cannot be due to any strictly electrochemical process, since such chemically opposite gases as hydrogen and oxygen produce the same result, as does also the inert gas nitrogen. The fact that un-ionized organic compounds in solution do not alter the action of water appreciably, and that weak, (slightly dissociated), electrolytes do not reverse the effect as strong electrolytes do, shows that the action of substances in solutions is due to their ions. The logical conclusion is that the charge on the gas is caused by its selective adsorption of ions from the solution, the sign of charge depending on whether the positive or the negative ions are adsorbed more strongly. The existence in the gas of both positive and negative carriers of charge has been shown e~perimentally.~ Most interesting js the experiment of Aselmann4 in which he found that the gas bubbled through a NaCl solution contained both positive and negative carriers, those with positive charge containing Na, as shown by a flame test, the others beilig free of Na. This is exactly what would be expected on the basis of adsorption. The gas would adsorb both positive and negative ions, the concentration of the ions determining which was adsorbed more. Becker: Jahr. Rad., 9, 52 (1912). Lenard: Ann. Physik, (4) 47, 352 (1915). 3 Bloch: Ann. Chim., 22, 370; 23, 28 (1911). 4.4selmann: Ann. Physik, 14), 19, 960 (1906). 2

882

HERMANN F. VIEWEG

The process by which the gas acquires its charge is quite simple. As long as the gas and liquid are in contact, adsorptive forces hold some of the ions of the solution to molecules of the gas. The bubble in the liquid, or the gas surrounding a liquid drop or surface, will have a charge at the boundary. If then, due to some sudden mechanical disturbance, either the bubble forces its way out of the liquid, or the drops of liquid are broken into finer drops, or the liquid films on the surface of a solid are torn apart and pulverized, the gas is violently separated from the liquid, and in the process carries with it some of the adsorbed ions. The extent to which any particular ion is adsorbed is, of course, dependent on its concentration. Although sufficient data were available to show what certain ions would do, no record had been made of the action of certain others. For this reason it was desirable to measure the effects of various solutions not previously investigated. The method of bubbling gas through a liquid was chosen as being more accurate and convenient than that of testing solutions by rubbing them on the surface of solids. Air was used as the gas, since this is the atmosphere that is normally present in experiments. A few solutions, whose action in connection with hydrogen and oxygen had been examined by Mozer, were tried to show the comparison between air and other gases.

Experimental In determining the charges developed by bubbling air through various liquids, use was made of apparatus very similar to that described by Coehn and Mozer. To purify the air it was passed through KOH solution to remove COz and then through H2SO4to dry it. The reason for removing COZwas to prevent changes in dilute alkaline solutions during prolonged bubbling. The purified air was then passed through a grounded copper tube containing fine brass gauze, to remove any charge it might have acquired. The solution itself was contained in a flask, this part of the apparatus being like that used by Mozer. Just above the opening of the side-arm leading from the flask was suspended a perforated platinum cone covered at theb ase with 80-mesh platinum gauze, The cone was connected to one set of quadrants of the electrometer, by means of wires insulated and shielded as described before. The cone and the flask containing the liquid were enclosed in a grounded metal container. As gas bubbled through the liquid and went out through the side-arm it gave up the charge it acquired to the gauze, the electrometer deflection being proportional to this charge. In order to compare all the experiments on the same basis, measurement was made in each case of the change in deflection during the 15 minutes from 5 min.-20 min. after commencing the bubbling of the gas. During the first few minutes the action of the instrument was usually erratic. Since the value of scale divisions in terms of quantity of charge may vary from day to day, a solution of . I N NaN03 was used daily as a standard of

883


comparison. For this reason all the results recorded are calculated with reference to the effect of . I N N a N 0 3 as unity. The rate a t which it is desirable to bubble the air depends on two considerations. First, if it is done too slowly, the rate of development of charge is not great enough to make measurements accurate. However, if it is too rapid, particles of liquid may be carried along and strike the gauze, causing erroneous results. The rate of flow was varied until a satisfactory speed was found. This was maintained throughout all the experiments. Great care had to be exercised to guard against the presence of any impurities in the solutions. The smallest traces of some substances will cause results to vary greatly from their true values. For example, a very small

-12

-20

-8

6

- 4

-2

0

2

C7LARG.E ON AIR FIG.I Variation in Charge on Air after bubbling through Solutions XaOH, SaKOa, and “03.

amount of acid in a salt solution will lower the inversion concentration considerably. Therefore the flask was thoroughly rinsed several times when changing from one solution to another. Also the flask and other glassware, such as pipettes and graduates, were cleaned with bichromate-sulfuric acid mixture every day before using. In this way, it was possible to obtain consistent and reproducible results. A tabulation of the results obtained follows. The concentrations are those of ionized compound in solution, these being calculated from conductivity data. Concentrations were expressed in this manner because the object of getting the data was to show the variation in adsorption of the ions with change in their concentrations. Determinations were usually made of the charges on the gas after bubbling through I , .I, .OI, and .OOI N normal solutions. I n some cases other concentrations were also found necessary, From the charges so obtained, the prob-

HERMANN F. VIEU'EG

884

TABLE I1 Charge on Air after bubbling through Solutions Effect of .I KaN03 taken as unity. Nan'O NaOH Charge N

pu'l

Charge

.OOI

-10.6

,001

-11.3

.OI

+

-

1.17

.OI

+

1.00

.I

1.5

I

+ 0.87 +

.I I

-

5.0

1.45

Inv. Conc. = .018N

Inv. Conc. = .03I ,N

NaCl

KOH

,001

-10

.OI

-

1.3

+

0.91

+

.I I

1.34

,001

-9.1

.OI .I I

-4.4

Inv. Conc. = ,023 N

+I. $1.6

Inv. Conc. = . 0 2 5 N HC1

"03

.OOOI

,0001

-6.0

.001

.OOI

+O.ZI

.OI .I

$0.73

.I

I

I

+I.41

.OI

Inv. Conc. = , 0 0 0 3 N

+I. I2

Inv. Conc. = . O O O ~N

Th(NO3)4

KgFe(CN)6

N

Charge

N

Charge

. 001

-0.61

.001

-15

.OI

$1.71

.OI

- 6.8

.I

+7.9

.I I

+ +

Inv. Conc. = . OOI 5 N

0.41 0.82

Inv. Conc. = .06 N

*

BaClz

. 00I

-8.1

.OI

0

.I

+I.IO

I

+I44

HzO (distilled)

- I50

Inv. Conc. = . 01 N Concentrations refer to concentrations of ions of the substance. Thus, . I 19 ?i NaCl is about 84y0 dissociated, and therefore is .I N with reference to Na and C1 ions. In the cases of Th(lu'03)4andK3Fe(CN),, the concentrations given refer to the normality of the solution in terms of total dissolved salt, and not merely the ionized portion. *Owing to the abnormally high value of the charge due to water, its relative value with reference to the others is only approximate.

885


able inversion concentration was estimated, A solution of this strength was then tested, and using the results of this with the other determinations, the inversions concentration was determined graphically. The results are given in Table 11. Three curves, showing the variation in charge on the gas with concentration of ionized HKO,, NaN03, and KaOH are shown in Fig. I. These compounds were chosen as examples of an acid, a salt, and a base.

Discussion The curves shown in Fig. I are typical of the actions of all well-ionized electrolytes. In dilute solutions they impart to the gas a negative charge, which decreases rapidly with increasing concentration. At a definite concentration, the gas acquires no charge; above this the gas becomes positively charged, the magnitude of charge increasing only slowly with the concentration beyond this point. Mozer's curves are similar to these, although, of course, the relative values of charges at different concentrations, and the zeropoint are somewhat different. This is due partly to the use of a different gas, and also to the difference in experimental conditions, such as the velocity of the gas, and the size of orifice through which it entered the solution. The first point that strikes one on looking over the data is the relatively high negative charge due to water itself. This means that at concentrations as low as IO-' the OH- ion is adsorbed very strongly: H+ practically not a t all. (Mozer could find no positive carriers in the gas after it had bubbled through pure water). In the case of solutions of electrolytes, it is therefore to be expected that the ions of water will have an effect which will be to make the gas negative. Let us consider some examples of solutions of electrolytes. The curves shown for HX03, KaN03, and NaOH are typical for acids, salts, and bases. The action of a solution on a gas is a resultant of the combined actions of the ions of water and those of the solute, The ions of the water, as just stated, due to the much greater adsorption of the OH- ion, tend to charge the gas negatively. The effect of the presence of the ions of the electrolyte is to change the charge in the positive direction. As the concentration of the solute increases the relative importance of the water decreases. Thus in .OOI N solution the water ions are outnumbered IO,OOO to I ; in I N solution, IO,OOO,OOO t o I. In the latter case the water ions must play a much less important part than in the former. The charge on the gas should therefore be a resultant of a negative component, decreasing with increased concentration, and a positive component, increasing with the concentration, In examining the charge on hydrogen after bubbling through K N 0 3 solutions, Mozer found the following distribution of positive and negative carriers: Pos. Neg. Conc. .OOI 5 50 .OI

I3

22

.I

I5

I1

886

IIERMANN F. VIEWEG

The negative charge is due to the sum of the adsorption of OH-, decreasing as the concentration of solute rises, and of the negative ion of the electrolyte, increasing with the concentration. The displacement of OH- by another anion results in the decrease of the adsorption of negative ions. The positive charge is the resultant of the increasing adsorption of the cation, and the practically negligible adsorption of H+, and therefore increases with the concentration. The total result is to make the charge more positive as the solution contains more of the electrolyte, With alkalis or acids, the situation is somewhat different from the case of a salt. In alkaline solutions, the only negative ion present is the OH- ion. The negative charge on the gas must therefore increase with the concentration. The adsorption of H+ is negligible, As the concentration increases, the adsorption of cation may increase more rapidly than that of the OH- ion, and at some point exceed it. This is found to be the case, but of course this concentration must be higher than that for all salts of the base in which the anion at low concentrations is less readily adsorbed than OH-. In acids the conditions are reversed, The concentration of OH- in solutions from .OOOI T\T up is small enough to make its adsorption of no effect. The negative charge will a t all strengths of solutions FIG.2 be less than in the case of salts of the Variation of Sdsorption by Air with acid, but it will increase with the conConcentration of Hydrogen centration. The inversion point for acids and Hydroxyl Ions. will therefore occur at lower concentrations than are necessary for salts of the acid. It should be noted, however, that this would not necessarily be due to the adsorption of H+ being greater than that of other positive ions, but might be due essentially to the decreased negative effect of OH- ions in acid solutions,

It may be well to point out why the difference between the inversion point for acids and that for salts is so much greater than the difference between inversion points of salts and bases. For examples these concentrations are for HN03 .0003 N, for NaN03 .o18 N, and for XaOH ,031 N. When we consider the probable relations between adsorption and concentration for both the OH- and the H+ ion, the reason is apparent. The data indicate that the adsorption isotherms for these two ions are qualitatively as shown in Fig. 2 . This is based on the assumption that a t a concentration of IO-^, OH- has reached nearly its maximum of adsorption, this increasing very slowly beyond this concentration. H+ a t a concentration of IO-^ is practically not adsorbed at all. The evidence is that a t normal concentration the hydrogen ion is adsorbed more than the hydroxyl ion.


887

It will be obvious that while in acid solutions the effect of the otherwise strongly adsorbed OH- is cut down very much, and the effect of H+ increased considerably above its value at I 0-7 concentration, in alkaline solutions the increase in OH- adsorption is not so great, and the effect due to removing the already negligible H+ is nothing. Hence the acid inversion point should be much further below that of the salt, than the latter is below the alkali zero-point . The question may arise as to why the OH- ion of the water is considered as having any effect in a salt solution where other anions are present in so much greater number. The large negative charge due to pure water shows that the adsorption of the hydroxyl ion is enormous in concentrations as low as IO-’. So although in a salt solution there are many more negative ions present, trying to be adsorbed by the gas, the OH- ions are much better at it, and will therefore have an appreciable effect. Of course, the greater the concentration of the salt, the more will its anions displace the OH- ion. Some other interesting information can be obtained from these data. For example, BaClz changes its action a t .OI N, whereas NaCl must be . 0 2 3 N before it changes. This means that the divalent Ba++ ion is adsorbed more strongly than the monovalent Na+. Also the tetravalent Th++++ is even much more effective, showing that it is very strongly adsorbed. This jndicates increasing adsorption with increase in valence. On the other hand, the trivalent anion Fe(CN)6--- is even more strongly adsorbed than OH- and therefore the inversion for ]K&(C”)6 is higher than for KOH. It is reasonable to assume that in normal concentration the effect due to the ions of water does not amount to much, and that therefore the relative charges give a measure of the relative adsorptions of the various ions. On this basis we find that the order of adsorption from greatest to least is: Th++++,H+, Ba++, K+, Na+, Fe(CN)6---, OH-, C1-, NOa-. Applications to Frictional Electricity One of the objects of this investigation has been to see how the observed effects of moisture account for anomalies previously observed. It is evident that since certain films on the surface of a solid tend to add a negative charge while others tend to add a positive one, the possible variations of effects are many. For example, the normally positive surface might have on it a film which would acquire a negative charge, while the other substance was covered with a film tending to become positive. The results obtained in this case would vary with the extent to which the two solids were rubbed together. KOdoubt much of the confusion in the past has been due to effects of this nature, To consider an actual case, Shaw found that mica when beaten lightly on glass became positively charged, when it should have become negative. When rubbed firmly the mica charged negatively. This is what would be expected if the mica, (which has a strong tendency to adsorb moisture), were

888

HERMANN F. VIEWEG

wet, If a clean, dried mica surface be used, it becomes negatively charged even if only rubbed light,ly. I n some experiments on sand, Phillips'found that if it were allowed to slide over a filter paper, the paper charged positively when it should charge negatively. He explained this on the basis that sharp edges of sand will charge the paper positively, while dull surfaces of sand would cause a negative charge. There is no apparent reason why the paper in this experiment came into contact mainly with the sharp edges of the sand. It is interesting to note that Phillips considered moistness of the paper a prerequisite for the experiment.2 It is to be expected that if the paper were sufficiently moist, it would acquire a positive charge when rubbed by the sand, His experiment has however not been repeated here. During a previous in~estigation,~ carried out in this laboratory, some unusual effects were noted when gases were bubbled over metal electrodes in acid solutions,

It was found that, using oxygen on one of two copper electrodes, the aerated one was anode, contrary to expectations. Iron or nickel electrodes acted as they should; they were cathode when oxygen was bubbled over them. One should also have expected that hydrogen would reverse the results, but actually it acted in exactly the same way as oxygen; copper was anode, and iron or nickel cathode when the gas was bubbled over them. Similar results were obtained with nitrogen, air, illuminating gas, and carbon dioxide. No attempt will be made here to explain why copper acts differently from iron and nickel. But the point of interest is that the charge producing these effects must be due to the same phenomenon that we are discussing here; namely the charge acquired by a gas due to selective adsorption of the ions of a solution. This indicates at once that the potential of a gas electrode may be affected somewhat by this action. In a recent paper by A r k a d j e ~it , ~is shown that the potential of a hydrogen electrode with reference to a . I N HBr solution is increased, that is, made more positive, by the addition t o the solution of a neutral salt. With KN03 and KC1 the lowest concentrations made the electrode more negative; then with increasing concentration it became more positive. With iSaC1, LiC1, KBr, and LiBr, the action was to make the electrode more positive with greater concentration. It seems reasonable that these results are due, at least in part, to the adsorption phenomena discussed in this paper. Phillips: Proc. Roy. Inst., 19, 742 (1910). "I need hardly point out that the filter paper used should not be specially dried. Pieces which have been left about in a room for a few hours adsorb sufficient moisture to insure the right degree of condurtivity." 3 Bancroft: J. Phys. Chem., 28,842 (1924). 4Arkadjev: 2. physik. Chem., 104,192 (1923). 2


889

I n this study of the effect of moisture, certain anomalies observed by previous workers have been explained; and the possible explanation of others not discussed is thereby suggested. The necessity of working in the absence of moisture, if reproducible results are desired, is apparent.

Summary The results of this investigation have been the following: I . A frictional electric series has been established, including several substances not previously placed, and showing the effect of using different crystal faces. 2 . An explanation of frictional electricity has been proposed, using the electronic structure of matter as a basis. 3. A suggestion as to the physical significance of “Coehn’s rule” has been offered. 4. The effect of moisture films on frictional electric charges has been shown t o be related to Lenard’s “Wasserfallelektrizitat”. 5 . The charges produced when air is bubbled through various solutions have been measured. 6. An explanation of these effects, (4 and 5 ) , has been presented on the basis of the selective adsorption of ions by a gas. The writer wishes to express his sincere appreciation to Professor TV. D. Bancroft, under whose direction this investigation was carried out, for his valuable criticisms and helpful suggestions, Cornell Unicersity.

Frictional Electricity

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