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XXXVI. The Clark Cell when producing a Current.
To the Editors of the Philosophical Magazine.
you published a letter from Professor Threlfall criticising my experiments on the Clark cell when producing a current. I should like to reply to some points in it.
Professor Threlfall quotes my definition of the quantity which I have called the polarization of the cell. The method of determining this quantity depended on measuring separately the resistance of the contents of the cell treated as an electrolyte and of the wire through which the cell was used to maintain a current. Knowing the current, I calculated the E.M.F. required to maintain it in an equivalent wholly metallic circuit having a resistance equal to the sum of these two values. The difference between this E.M.F. and that of the cell at rest I tabulated as the polarization with varying current-density. This point was considered very carefully by me at the time, and it appeared that the resistance of the contents of the cell could only be changed through unequal migration of ions and electrolytic endosmose. Now the effect of these two causes would be permanent when the cell was again placed on open circuit; and so if measurements taken before and after the cell was used did not show any wide divergence, it is legitimate to assume that whilst the current was flowing the resistance would lie somewhere between these two values. This is the reason which led me to adopt this method of statement of results.
The actual measurement of the resistance of the cell showed, as I expected, very little change. The testing-current was of the order of .007 ampere.
The temperature of the 147 ohm wire through which the current of approximately :01 ampere was maintained could not have been far different from that of the oil-bath. A current of this magnitude would only produce 12:6 gram Centigrade thermal units in one hour; and as the wire (20 grams) was openly wound“ on an ebonite frame,” it does not seem likely that it would have been much hotter than the oil. It would require a rise of 10° C. to produce a change of •6 ohm. The case is not at all comparable with Mr. Griffiths', where current up to an ampere was used. Besides, it must be remembered that any possible error in the resistance of this wire would have very little influence on the magnitude of the quantity which I have called the polarization of the cell. My results when the current was maintained indicate that the potential-difference at the electrodes became less. In my earliest experiments, when two Leclanché cells were used in the potentiometer circuit, I found an effect similar to that observed by Professor Threlfall. This is stated in my paper, § 5 and $ 7, and as such a result was not expected and did not for many reasons seem to be connected with the Clark cell, I placed large Clark cells in the place of the Leclanchés.
The considerations which led me to make this change may be shortly summarized. Firstly, the comparison of the size of the plates in the two cells is greatly in favour of the large Clark, and also the soluble depolarizer in that cell would cause more efficient depolarization. Secondly, I have frequently observed when using the potentiometer that the same value may be obtained for the potential-difference of a shortcircuited Clark before and after an interval during which the Clark has been on open circuit, provided no large plug change has been made during the interval.
On the other hand, a large plug change, such as testing the E.M.F. of the cell with its poles open will lead to a new value when its poles are again closed. Thirdly, when determining the absolute value of the E.M.F. of the Clark cell with Mr. Glazebrook (Phil. Trang. 1892), we thought it safer to avoid large plug alterations, and so abandoned this process to adopt one in which a mercurial rheostat in the main circuit could be varied so as to maintain the electrodes of the standard resistance at a constant difference of potential. With this alteration my experiments became regular ; seven sets were obtained (one lasted 16 hours), and I was satisfied that the original irregularity was due to the Leclanchés. The average of four sets is given in my paper. If I were to repeat these measurements I should now use one large accumulator and a wire rheostat for that part of the potentiometer where alterations of resistance would have to be made, and thus avoid all rapid plug changes. This arrangement would, I am sure, give better results.
My object in referring to Mr. Fitzpatrick's paper in the easily accessible B.A. Reports was to avoid a lengthy description of an apparatus which is in continuous use in the Cavendish Laboratory. In his paper Mr. Fitzpatrick has made the proper references to Macgregor's and Professor Kohlrausch's well-known work.
I am. Gentlemen,
S. SKINNER. Cambridge, March 9, 1895.
XXXVII. On some Experiments with Alternating Currents.
By A. SADOWSKY, University Professor in Turiew (Dorpat), Russia".
To the Editors of the Philosophical Magazine. GENTLEMEN, N No. 238 (vol. xxxix. 1895) of your valuable Magazine
Mr. Griffiths has published a paper on “Some Experiments with Alternating Currents,” in which he describes some experiments with Lenard's bismuth spiral. I was investigating the same subject in 1892–1893, and obtained the following results, agreeing well with those of Mr. Griffiths.
(1) With the bismuth spiral in a strong field absolute silence in the telephone is never obtained, but only a minimum of sound.
(2) The superficial distribution of alternating currents is without influence on the resistance.
(3) The resistance of bismuth depends on the phase of the current.
The following results are similar to those of Mr. Griffiths, bat do not coincide with them in every respect:
(1) The bismuth spirals have something like the selfinduction, but not equivalent to it; I have tried tlie hypothesis
where r, is the resistance for a constant current,
i the current,
bridge and telephone). Under this hypothesis r must be always >ro, which does not agree with the experiments of Lenard, Zahn, and myself.
(2) I have observed the difference of resistance with frequency 500 without field to be as stated by Messrs. Lenard and Zahn; the resistance with a constant current was greater (0:1 per cent.).
In addition to the above, I have found the following:
(1) M. Lenard's statement that the difference of resistance is due to the currents of great frequency, i. e. 10,000, is
Let ordinates of the curve A B C D represent the
* Abstract of a paper published in the Journal of the Russian PhysicoChemical Society, vol. xxvi. pp. 81-156 (1894).
current and abscissæ the time. If we divide the curve in three parts AB, BC, CD, such that AB,=B,C1=CD (approximately), and measure the resistance with the currents
AB, BC, CD; then denoting the measured resistance by P1, P2, P3, I have obtained experimentally :(a) Without magnetic field, with frequency 4–6:
Pi=p2=P3. (6) With intense magnetic field, with the same frequency, 4-6:
Pi>P2>P3. Taking P, as unity, the following figures are obtained for four spirals:
If we take the currents represented by the curves figs. 2 and 3 and measure the same P1, P2, P3, we obtain as above,
but the differences P.-P2 and pa—P3 are much smaller than with the current ABCD.
My method of experimenting for obtaining the data for 21, P2,
and P3 was similar to that of Mr. Griffiths. The alternating current was supplied by a small magneto-electric machine set in motion by an electromotor. This current was sent through a Wheatstone bridge, in one branch of which was placed a bismuth spiral. The galvanometer branch contained an automatic double intermittent contact (rotated by the magneto-electric machine supplying the alternating current), by which it was possible to complete the circuit of the galvanometer for the parts of the current AB or BC, and BC or CD, and thus to compare immediately pı
and P2 with
P3. In the first part of my investigation I measured without field the resistance of a bismuth spiral with a telephone, and
an alternating current obtained from a Kohlrausch inductorium. As interrupter I took a toothed wheel turned by an electromotor; the frequency of interruption varied between 92 and 2088, the difference of resistance varied with the frequency between 0·12 and 0:27 per cent.
March 18, 1895.
XXXVIII. Notices respecting New Books.
JAMES GEIKIE, D.C.L., LL.D., F.R.S., 8c. Third Edition.
8vo. Stanford : London, 1894. THIS THIS new edition of an important geological work has been
“ largely rewritten" by the Author, and contains two new chapters on the “ Glacial Phenomena of North America," by Professor T. C. Chamberlin.
As a systematic account of the visible results of ice-action on a large scale in this part of the World, and of the probable history of the origin, progress, and end of this period of glaciation, we have here a valuable repertory of facts and opinions recorded by many geological enquirers and grouped by one who has studied the subject for almost a life-time. Formerly one of the members of the Geological Survey engaged in mapping the geology of Scotland, and now holding the Chair of Geology at Edinburgh, Professor James Geikie has naturally taken up one of the most interesting groups of geological features in that country as a