STUDIES OK OT’ERS’OLTAGE. 1-11 ELECTRODE DISCHARGE PHEXOMETA STUDIED BY ~ I E A T OFSAN ELCCTROMAGSETIC ISTERRVPTER TOGETHER KITH AS OSCILLOGRAPH’ A . L. FERGUSON
ISD
G. 11. CHEN
Department of Chemistry, Cxiicrsit!j of Michigan, Ann Arbor, Michigan
ReceiLed February 23, 1934
I n previous articles (1) the authors have studied polarization phenomena by nieans of a conimutator in connection n i t h a potentiometer, also by nieans of a commutator in connection with an oscillograph ( 2 ).z Cornparisons were made of the direct and commutator methods for measuring such phenomena. The results of all tlie earlier n-ork indicate that both methods give identical values for electrode potentials when measurements are properly made. This is contrary to the findings of most other investigators in this field. It is generally stated that the conimutator method gives lower values than tlie direct. The difference between the two is attributed to a part of the voltage measured b y the direct method being used up in driving the current through some kind of a resistance other than electrolytic resistance a t the surface of tlie electrode. This resistance is coninionly referred to as “transfer resistance” or “surface resistance.” The existence of such a resistance has been postulated for about one hundred years. If a surface resistance exists, then, of course, a part of the applied potential, which is the thing measured by the direct method, is used up as an I R drop a t the electrode-solution interface, and, naturally, the electrodes themselves are not charged to so high a potential. The back potential, which is measured by the commutator method, must be less, therefore, than tlie value n-hich the direct method gives by an amount equal to the I R drop through the film on the electrode. It is assumed, of course, that account is taken of the ordinary I R drop through the solution. One of the most active supporters of the transfer resistance idea is E. Newbery. H e has made about fifty thousand observations with the rotat1 The oscillograph used was purchased with a grant from the Faculty Research Fund of the TJniversity of Michigan, nhich thus made this work possible. * Since the publication of this article, our attention has been called t o two articles by S.K. W’aldorf in which he made use of an amplifier-oscillograph arrangement similar t o ours in his study of dielectrics (Physics 3, 1 (1932); J. Franklin Inst 213, 605 (1932)). 1117
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ing conimutator and about two thousand I\ ith the cathode ray oscillograph during a period of about twenty years. It was sho.irn in earlier papers by tlie autliors that most of the nieasurements made by a rotating coinmutator are averages and therefore practically valueless so far as furnishing information upon tlie question of transfer resistance is concerned. Those ~ l i defend o the transfer resistance idea base their support largely upon an instantaneous drop in electrode potential, almost universally observed, inimediately after the polarizing circuit is opened. As Sen-bery states in a recent article (3), “ K e n-ill first utilize tliese curves for a discussion of the nature and properties of tlie very controversial ‘transfer resistance.’ The most striking feature of each of tliese curves (lie refers here to electrode discharge curves obtained n-itli a cathode ray oscillograph) is the clear gap n-liicli appears luetn-een the upper and lon-er sections. The existence of tliis gap proves conclusively that tliere is an instantaneous fall of potential at tlie electrode the moment tlie exciting current is interrupted. . . . This resistance-transfer resistance-may be measured by observing the vertical height of the gap in the curves, calculating the corresponding change of E.M.F., and from this, calculating tlie resistance, tlie total current flowing being knoirn. When this is done, it is found that although the gaps in the curves are n-ider a t tlie liiglier current densities, tlie actual transfer resistances are lon-er. In fact, transfer resistance increases rapidly as tlie current density is lowered, and a t very low current densities it is frequently above 100 ohms per sq. mi.” In an earlier article (2) tlie autliors have presented results obtaiiied with a rotating commutator in connection u-ith a moving coil oscillograpl~that give no indication of an I R drop, except that due to the I R drop tlirougli the solution, n-hen tlie charging circuit is suddenly opened. The true nature of the drop in potential which talr‘es place imniediately after the polarizing circuit is opened could be determined niucli better if the curve representing such a drop could be superimposed upon another curve known to be due to an I R drop only; or on one known to lie due to both an I R drop and the falling off of a true electrode potential. In the work to be described such an arrangement was used. The desired result was accomplished by the substitution of an electromagnetic interrupter in place of the commutator. The circuits involved are represented in figure 1. The portion of the figure within the circle and labelled “Drum” represents the face of the rotating film drum. Supported in front of the drum are three pairs of bronze brushes, Brl, Brz, and Brg. The pair of brushes Brl is short-circuited once during each revolution of the drum through the contact P,. I n a similar manner the pair Br2 is short-circuited through Pb. The contacts P, and P b are permanently attached to the face of the drum, but contacts PC1,Pc3,etc., are removable
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and only one is used a t a time. The portion of the drum included in the angle cc is not covered by the film. When the Brl brushes are shortcircuited tlie electromagnet A closes the circuit through the lamp L of the oscillograph. Just follon-ing this the brushes Br? are short-circuited through the contact P,in some one of the positions 1, 2 , 3, etc. This closes the circuit tlirougli the electromagnet C, which opens the polarizing circuit I , . I n this iiiaiiner the start of the discharge interval can be located a t any one of several points throughout the length of the filni. After the entire length of the film has been exposed tlie lamp is turned off by means of the
FIG. 1. CIRCUITS FOR
ELECTROMIGSETIC ISTERRUPTER
liruslies Brz as they make contact a t P b , and thus the circuit through electromagnet B is closed. As described in the previous article ( 2 ) , the potential to be studied may first be nieasured with a potentiometer, which gives the value by the direct method; it is then applied to thc grid circuit of the amplifier, and the voltage VI adjusted until the plate current as indicated by the ammeter is at some value near the upper part of the linear portion of the amplification curve. The plate current is then reduced to any desired value by the balancing potential V,, so as to bring the line on any desired part of the width of the film. Although potentials of different magnitudes are applied to the grid circuit of the amplifier, it is possible by mean3 of the
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variable voltages VI and V2 to cause the same current to pass t h o u g h tlie oscillograph. Under such conditions the lines on the film which correspond t o the various potentials are superimposed up to the point where tlie polarizing current is interrupted, and, if the contact P,is in the same position on the drum face, each potential u ill be interrupted a t the same point. The relative natures of the various discharging potentials may thus be directly determined. In order to inake a coiiiparisoii between an I R drop and depolarization, pictures were taken of three types of potentials: (1) single electrode potential against its ow1 reference electrode, n liicli is either a pure potential or a pure potential plus a potential over transfer resistance; ( 2 ) single electrode potential against the opposite reference electrode, whicli is a pure potential plus the I R drop through the solution and plus an I R drop due to I
7
3
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*
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~ FIG.
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2.
~
~
~
*
2’
*
A
DISCHARGE CURVES O F V A R I O U S
A
A
*
*
~
TYPES
A = potential of anode against anode standard; AI = potential of anode against cathode standard; I = potential of anode standard against cathode standard.
transfer resistance, if any exists; ( 3 ) the potential between the reference electrodes, which is due o d y to the I R drop through the solution and any transfer resistances a t the electrode surfaces. The same electrolysis system was used as described in a previous paper; the electrodes were platinized platinum and the solution 2 N sulfuric acid. In figure 2 are three curves: A sliows tlie drop in potential of tlie anode as measured against the anode standard; AI s h o w the drop in potential of the anode as measured against the cathode standard, and includes, therefore, the I R drop through the solution; and I shows tlie I R drop through the solution only. The point d is the common breaking point for all the curves. The line to the left of d is heavy because it is cornnion to all three curves. It is universally accepted that tlie potential over a pure resistance should
A
S T U D I E S O S OVERVOLTAGE. TI1
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drop instantaneously. Curve I , u-hich pictures such a drop, is not vertical as it should be to shox an instantaneous drop; this results froni a slight lag in the vibrating element of the oscillograph. There can be no doubt, however, that A does not slion- any drop due to a resistance, n-liich means there could have been no drop due t o transfer resistance a t the surface of the electrode included in the original potential measured by the direct method. It is clearly evident, on the otlier hand, that AI does contain a drop due to resistance. The original potential of Ar contained all of the I R drop slion-n by I , and froni the breaking point, d, the two curves are identical for some distance, and evidently would have remained identical down to the horizontal portion of I had it not been for a slowing-up of the vibrating elenient as it approached the end of its return swing for curve I . Another way to show that A I contains the same I R drop as I , and no more or no less, is to measure the vertical distance between A and 9, nliicli is
h found a t all places to be equal to tlie total drop in I . Such a test was applied to several sets of curves and aln-ays held. To show the magnitude of the voltage changes, tliree constant voltage lines, 1, 2, and 3, v,ere drawn across the film. The distance betiyeen 1 and 2 or bet\\-een 2 and 3 is 40 millivolts. By means of the tinling wive, which is for a 120-cycle current, the rate of fall of anode potential may be determined easily. To learn something more definite ahout the lag of the vibrating element, a series of discharge curves, over a pure resistance, were taken u-it11 the same potential applied to tlie grid but \Tit11 different balancing potentials so as to bring the curves a t different positions on the width of the film. These curves are slion n in figure 3. The contact point, P,, n as inserted in different positions so a5 to cause the break to appear at successive positions longitudiiially on the film. The first curve was taken at tlie highest speed and the last a t the leu-est; the speeds are represented by tlie 120-cycle tirn-
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ing waves a t tlie bottom. These curves show tilat wliat is really an instantaneous drop is not recorded on the film by 3 vertical line. In figure 4 is shown a series of curves obtained with the same film speed, but for different resistances. These all represent potential drops due to pure resistances only, and show similar characteristics. The rate of drop is the same a t the start, but slows up distinctly, and in about the same manner, as the element approaches its zero position. From the curves in figures 3 and 4 it would be possible to calculate the lag under a given set of conditions and apply it as a correction. It is evident from these oscillograms that when a curve produced by this oscillograph is aiialyzed to determine whether it contains a potential due to an I R drop, this lag of the vibrating element and variation of tlie lag with speed of film and displaceinent of element must be taken into account. The best way to do this is
AFIG.
A-
Anzk A
A A A
4. POTENTI.4L DROPS OVER A S E R I E S O F P U R E RESISTAXCES
For curve 1, R = 10 ohms; for 2, R = 15 ohms; for 3, R = 18 ohms; for 4, R = 20 ohms; and for out 5 millivolts.
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5 . No curves that represent tlie potential bet~veeiian electrode and its on-ii reference electrode, i.e., that contain 110 knon-n I R drop, show any indication of an I R drop due to surface resistance. 6. This work substantiates the conclusion arrived at in several earlier papers from this laboratory, that there is no transfer or surface resistance at platinized platinum electrodes in 2 3' sulfuric acid. The junior author wishes to ackiion-ledge his indebtedness to t'he China Foundation for tlie Promotion of Education arid Culture, for the granting to him of a fellowship which made it possible for him t>ocontinue his research Ivork at tlie University of llichigan. REFERENCES (1) FERGI-SOS .ISD CITES: J. Phys. Chem. 36, 1156, 1166 (1932). (2) FERGUPOS .ISD CHES: J. Phys. Chem. 36, 2437 (1932). (3) SETRERT: Trans. Am. Electrochem. Soc. 68, 187 (1930).
