STUDIES ON OVERVOLTAGE. V* A Moving-Coil Oscillograph Commutator System for the Study of Overvoltages and Transfer Resistance‘ BY A. L. FERGUSON AND GRAHAM M. CIIEN*
I n previous articles’ it was shown that difficulties inherent in the commutator rendered the commutator-potentiometer system undesirable for the measurement of overvoltage and transfer resistance. The present article is a description of a commutator oscillograph system which is much superior for such determinations. The moving coil oscillograph has been used by Rei~hinstein,~ LeBlanc? Latterye and Miller’ in studies of transient phenomena at electrode surfaces in electrolytes. In all these cases a source of intermittent direct current was used and the electrodes were connected directly to the oscillograph. The method was finally rejected because the oscillograph drew considerable current for its operation and thus changed the potentials of the electrodes under investigation while those potentials were being measured. This difficulty was partially overcome by Hollers through the use of a vacuum tube amplifier. He used the new arrangement to study polarization and the nature of “transfer resistance” or as he called it “boundary resistance.” His oscillograph required 1 5 0 milliamperes for full-scale deflection which made it necessary to use a three-tube amplifier. Even then it required 1 5 millivolts change in grid potential to produce a change of I milliampere through the oscillograph circuit; which means the arrangement is not suitable for accurate work. The cathode ray oscillograph has been adopted by Newberyg for the study of polarisation and overvoltage. It is ideal in respect to its capacity to accurately follow extremely rapid changes of potential. Vibrations having a frequency of 1 0 6 per second may be recorded. It is also especially desirable in that it draws practically no current from the source. A marked disadvantage, however, is that it gives a deflection of only one millimeter per volt. By the use of an amplifier it is possible to secure deflection of one millimeter per IOO millivolts. Another very serious difficulty is that the
* Contribution from the Chemical Laborator of the University
of Michigan. The oscillograph used was purchased from d e Faculty Research Fund of the University of Michigan which thus made this work possible. * The junior author wishes to acknowledge his indebtedness to the China Foundation for the Promotion of Education and Culture for the ranting to him of a fellowship which made it possible for him to continue his research wort a t the University of Michigan. a Ferguson and Chen: J. Phys. Chem., 36, 1156, I 166 (1932). Reichinstein: Trans. Faraday Soc., 9, 228 ($914); 2. Electrochemie, 16, 916 (1910). LeBlanc: 2. physik. Chem., 5, 469 (1890); Trans. Faraday SOC.,9,251 (1914). e Lattery: Trans. Faraday Soc., 19, 827 (1924). 7 Miller: J. Franklin Inst., 19,771 (1925). Bur. Standards Sci. Paper, 20, 153 (1925). Proc. Roy. Soc., 107A,487 (1925). 1
2438
A. L. FERGUSON A N D GRAHAM M. CHEN
energy of a single discharge of the cathode ray tube is so slight that many discharges are necessary to affect the photographic plate. I n fact Xewbery found that the plates had to be exposed for 30 sec. while the circuit through his electrode system which was being measured was made and broken about 14 times a second. I t is, therefore, a serious question as to how much confidence can be placed in values obtained with such an arrangement. This topic will be discussed more fully later. The Einthoven string galvanometer has been used by some to record electrode polarisation phenomena. Bowden and RideaP have used it extensively for this purpose. Concerning their work, however, Newbery states that “the maximum frequency of the string galvanometer, which is about 300 per second is far too low, and unreliable results have been obtained by using it for this purpose, as it is quite incapable of following the extremely rapid changes of potential which occur.” In order to determine with a high degree of accuracy whether there is any kind of a surface resistance over which a portion of the applied potential is lost before it reaches the electrode it is necessary to have an instrument : ( I ) that will give a complete discharge curve from the time of opening the switch; ( 2 ) that will show the path of the curve during the first 0.001second of the discharge interval; (3) that has a sensitivity of about one millimeter per millivolt; (4) that responds instantly to the variation of current through it; and (5) that draws no current from the electrodes under investigation. No apparatus or method thus far described in the literature fulfills these requirements. The present paper is a description of such an apparatus and met hod. Apparatus and Material The oscillograph used was a modification of the two-element Osiso made by Westinghouse Electric Co. The supersensitive element gives one centimeter deflection per milliampere. Energy to operate the element was supplied by a two-tube resistance coupled amplifier. A diagram of the amplifier is given in Fig. Ia. As the first tube, TI, a CX322 was used, and the second, Tz, was a power tube C X ~ ~ R1 I . and Rz represent two noninductive resistances of 850,000 and 50,000 ohms respectively, Rz being variable. The space charge grid g, was maintained positive 4.5 volts with respect to the filament, while the screen grid g2 was connected to the negative side of the potential under investigation and acted as a control grid. The grid g3 in the power tube was made 2 2 . 5 volts positive to the filament. Two milliammeters, AI and Az, were employed to measure the filament currents which were held constant by the variable resistances, rl and r2. The output current from the power tube could be measured by either the ammeter, Aml, or the oscillograph, osiso; see Fig. gb2. The amplifier was calibrated by applying known potentials to the grid circuit, and measuring the corresponding plate currents with a milliammeter. The amplification curve, plate current against grid potential is given in Fig. 2 . lo
Proc. Roy. SOC., 120A, 61 (1928).
STUDIES ON OVERVOLTAGE
2439
FIG.I The amplifier circuits are shown in Ia. The arrangement for connecting the unknown E.M.F. in series with a known potential VI, and the resulting potential to the potentiometer or amplifier is shown in rbl. The arrangement for connecting the output of the amplifier to either the milliammeter or the oscillograph is shown in Ibs.
2440
A . L. FERGUSON AND GRAHAM M. CHEN
For all experiments in the present investigation, the linear portion of the curves used is between the grid potentials, -3.564 and -3.724 volts, as indicated by the distance between the two vertical lines. This difference of potential, 160 m. v., gives rise t'o a plate current of 7.8 ma. The polarising current was interrupted intermittently with the direct connected motor driven commutator described in an earlier paper." The cell system and the switch board are the same as that described in an earlier article.12 The whole set up is a combination of t'he apparatus employed in the earlier commutator-potentiometer method with the addition of an amplifier and oscillograph. In Fig. Ibl, the symbol, G.A., for simplicity, represents the entire potentiometer assembly described in the earlier articles. By means of switches I, L, and LI in Fig. Ibl, the unknown potential, E M F . , may be measured either by the potentiometer as in the previous work or by the oscillograph. When the potentiometer is used, switch I is closed to the right and the other two are open; when the oscillograph is used, switch I is open and L and LI are closed. By means of the switch LIII a milliameter may be substituted for t,he oscillograph indicated in Fig. Ibz as osiso. The potential divider E, is used to insert a known volt'age VI in series with that under investigation in order to control the total potent'ial applied to the grid of the amplifier. Another potential divider E b is used to insert a known potential Vs in the plate circuit in order to control the current passing through the oscillograph. With this apparatus it was possible to measure potential changes of one millivolt and a time interval of 0.0001second. With slight modification the sensit,ivity could be increased even more if desired.
Experimental The manipulation of swit,ches for the various measurements when the oscillograph is used is the same as given in Table I of an earlier article.lz By setting the r.p.m. of the commutator at a definite ratio to that of the drum of the oscillograph, any desired number of complete cycles of the charging and discharging curve may be obtained on one film. The following symbols are used in connection with the various oscillograms. I . For measurements made by the commutator method. A = potential of anode against anode standard. Ai = potential of anode against cathode standard. K = potential of cathode against cathode standard. Ki = potential of cathode against anode standard. S = potential of anode against, cathode. I = potential of anode standard against cathode standard. Z and Z1 = zero lines for the two .vibrating elements. c = the beginning of charge. d = the beginning of discharge. 11 12
Ferguson and Chen: J. Phys. Chem., 34, 1156 (1932). Ferguson and Chen: J. Phys. Chern., 36, 1166 (1932).
Fro. 4 Single electrode potentials "I tho anode A' to A", end of the cnttiode K' to K" oliknined by tho commutnted-direet method st r.p.m.a. irorn zero lo Boo.
A . L. FERGUSON AND GRAHAM M.
*442
CHEN
was the r . p m of the commutator. The stnoot,h lines A' and K' represent espeetively with the commutator stalionthis anode :md cathode potent ary, which corresponds t,o the direct method. Lines A" and K" represent the potentials a t the r.p.m. 800. The lines beetween A' and K', and A" and K" correspond to intermediate r.p.m.s. Those oscillograms confirm the earlier conclusions that the electrode potentials decrease with incrcitse in r.p.m. of the zommutator due to increasc in brush contact resistance. T h e zigzag nature,of the curves is due to variation in contact resistance. The n cell were measured, xiso, at corresponding c u r r e ~ t sthroush the po the s a m ~t,imr, nnd the: dt&t.ashow a d in current with an incrtwr i n
FKi.
5
Tlrc ( I U I Y C S represent the potcetiirlr 01 t h e iollowing ' y s l e m (the ilurnerird v : h c s :we fur ihe direct meihod obtained witti the pvtcnlioinetcr). S = polmtiai of anode va enthocio slaniinrii. Bottom, r.Xq7Y;lop, 2.047V. A putentinl of anode ve anode titiwdiiitl. Iloftorn. i.rqzV; to!,, 1 . z i i V . K = pofentinl of rnthoclc VI eathotir stnndm. Boltom. 0.7oiV; toj', ~ 7 3 i Y . I =~ potential of anode stundiird VB crrthade 8t:wd:mi. Bottom, o.ooz, top, o.ojp. e I~ej~imiiiig ot ch;lrge. d = beginniiir of dirharyc. j=
~
r.p.ii1. for a constnnt
tot
lid; iis t h r r . p m iiicri to $ 2 0 nia. This d?cr
I.
I n :dl nieasuremcnts made with the oscillograpli the I.K. d r o p through the connecting wires, discussed in an iwlier p q m , was reduced to :L niiiiiiiium by using Iiravy lendn only six inches Song. Somc of tho oscillograms obtuincd with platinized pliLtiiiuiii dretrudrx i n 2 X ITSSO,UP shown iri Figs. 5 nnd 6. A largi. number of such oseillognms wcrc tnken undcr B variety oi conditions particu1:irly for as wide a range u€ currrni drwsity as possible 2nd for various spoeds of the commutiilw ;uid l h e film drum. The two shown here are typicill. I n t l i t ~figures C U P W A shows iho c l ~ m g ein potential of t h e anode during ehwgc anJ discharge; curvc IC, the change in cathode potential; curve S, thc clitrnge in tolul potential across thc crll; and I the I.lt. drop through the solution between the electrodes.
STUDIES ON OVERVOLTAGE
2443
Curves A and S are strikingly similar to those shown in Figs. 4, 5 and 6 of an earlier article,’2 obtained 80 laboriously by means of the commutator and potentiometer. These curves furnish strong confirmation of a conclusion &awn on the basis of the earlier work to the effect t,hat potentials measured aver the whale or any part of the charge or discharge intervals, as has heen done so frequently in the past by other investigators in this field, must he averages and practically meaningless. There are three sots of curves in Figs. 5 and 6; the set at the bottom of Fig. 5 was obtained with a current density of o.0005 amp.; the set at the t.op with a current density of 0.0089 amp.; the set in Fig. 6 with o.0221 amp. The lower part of S in Fig. 6 is
f
Fro. 6 T h e ct~rvct.i n this figure are for !,he &*meRyaterns as in €6. j except, miemti&. The -.due8 hr the direr! method are: S 2.172, A = I =
o.og7, and tlie poiarising current
=
o.ozzr amp.
-
missing because the total change in pot,ential a c r m ~the cell is grerstw than it is possible to record on the film. At the bottom of ihrse and all other oseiilogyrams to bo given liitfr is represented tho curve for a 1 2 0 cycle circuit,. It WM pointed aut in t,he earlier work (Fig. 5 , Ref. 5 2 ) that there W:LS an indication that lhc charging current reached a maximum early in t,he charge interval then decreased very slowly. There is a alight, indication of t,his same phenomenon in these I curves. This is shown much more conclusively in Fig. 7. The oscillograph proved to be an excellent means for making a dirrct comparison of the direct and commutator methods for measuring polarisation. Oscillograms were taken by both methods an the same film and practically at, the same time and thus under identical condit,ions. Typical photographs are given in Figs. 7 and 8. The records of potentids obtained by the direct mpthod appear as straight lines marked A’, K‘, I‘ and S‘. Tho numerical values for these potentials its measured by the potcntiometrr arc given under each figure. The curves obtained by the commufator method are designated in these figures with the refipective symbol8 without t.he prime& These curves demonstrate several points. Firstly, the curves for anode potential, A, by the commutator method show maximum values identical
Fro. 7 The euives s h ~ the n potentials a i YII~~OUR syeterna by both the direct and eommutnm nicthoda. A ’ = 1.335, K’ = 0.740, I’ = o.wj. current o.omz “mp. i
wiih the corresponding values, A’, by the direct mothod. This confinns thr ststmient mado by Ferguson and Van Zye‘$ that “the coininut,ator and the dirce! methods would give the same values if measurenlcnts hy t.he comrnutator method could be made u t the instant the polerising current is intermptcd.” This paint of view was greatly strengthened by i.hc work of ihr, present authors‘* with their improved commutator poteniiomcter; and oseillograrris shown here eliminate any quostion that might havc remainpd Stwmdl;-, t hr iiiariiiiuni potential of the c:iihodr by !hr a x o m h i o r inet,hod for low ciiirent iieiisitics, !hxn ihr vduc given t y t h t ii p h ~ m m e n n nwas not anticipnted, hui. ihem is no tlouht of iis riuliiy. I n magni!vdr thr diffrrmer :m,ounts t o !wo to {ivc riiiiliiolis. l‘his riiffwuiice can noi, Ix diii~t o ii ifiirisfor rr&!:inr:c siricc sncli a wrticnl &up nt thr bcginrring of dischmge and 1
difiiwv~et~ bctwrrii I hit inlrxiiniiiri v d u i ~by 1 IK COIIII~II-
Flu. 8 The euives ahow the putrntiitls .I various aystmns hy both the dirt-t end commutator methods. S‘ = ,.a@, K’ = 0.735, 1’ = 0.042, current = 0.010amp.
Ferguson nnd Van Zye: Trans. Am. Electrochem. Sa.,17,227 (rgzg).
STUDIES ON OVERVOLTAGE
2445
tator method and the value by the direct method as observed here. It should be noted that the total potential, S, across the cell measured by the commutator is less than the value given by the direct method by approximately the same amount as the cathode difference. The S curve is still rising and the probability is that K is also rising and, if sufficient time were allowed, the values by the two methods would become identical. This point of view is supported by other data not given here which show that at higher current densities the curves rise to a maximum much more rapidly and the values by the two methods are identical. There is a slight indication of that in Figs. 7 and 8. At the higher current density used in Fig. 7 the difference between the K and K’ curves is less than the difference in Fig. 8 where a smaller current density was used. The S curve could not be shown in Fig. 7 because the change in potential was too great to fit on the film, Thirdly, these curves show that the current I by the two methods is practically identical. As already pointed out there is a slight tendency for the current by the commutator method to be a little higher than by the direct method during the early part of the charge interval; but it drops off slowly as the charge interval continues. This is clearly evident in Fig. 7. The decrease in I is probably due to the corresponding increase in K. Fourthly, it is shown that time is required for the electrodes to reach an approximate maximum value. For the current densities of the order used in these experiments, this time is about 0.008 second for the cathode and 0.04 second for the anode. This means that at least that amount of time must be allowed charge and discharge intervals by any method used to compare the commutator and direct methods. Oscillograms not shown here were taken that prove this point. Fifthly and finally, these curves give no evidence of a vertical drop a t the beginning of the discharge intervals such as was found by Holler and Newbery, other than that due t o the I. R. drop through the solution. These curves confirm, therefore, all previous work done in this laboratory to the effect that there is no such thing as transfer resistance or any other kind of boundary resistance a t the surface of platinized electrodes during the electrolysis of a z N solution of sulfuric acid. summary I. An oscillograph-commutator system is described that is very well adapted to the study of transient electrode phenomena. 2. Oscillograms are given showing changes in various potentials during charge and discharge intervals. 3. Oscillograms are given showing a direct comparison of the polarisation potentials as determined by the commutator and direct methods. 4. The work with the oscillograph described in this paper confirms in every respect the work with the commutator-potentiometer system described in earlier papers.
