William Thomson to Faraday   12 November 1860

2 College, Glasgow | Nov 12, 1860

My dear Faraday

I enclose a specimen of the self recording electrometer’s work at Kew¹. This was done about two months ago, but some arrangements remained to be made before the apparatus can be set to act continuously. Mr. Stewart² writes to me that it will soon be permanently at work.

I should feel much obliged by your putting the photographic curve in its envelope again & posting it at your convenience. I am in no hurry to have it again however.

I was very sorry to give Mr. Vincent so much trouble about my abstract³. At the time I should have written it I was deeply in debt, for two papers which I had undertaken long before, and was in great difficulty as I can only write very slowly. I felt much obliged for your suggestion that the abstract might be deferred to the next Number: and I hope now to write it with ease & be able to send it very soon.

I have made as many attempts as I could during the summer to arrive at some more sure & accurate knowledge as to the electric relations between zinc, copper, and air. I have now got a way of experimenting which I think must lead to definite results, although I am far from seeing how to explain what I have as yet observed

The sketch represents very roughly my arrangements

A B is a metal case.

zz, cc copper & zinc discs as commonly used in Voltaic contact experiment.

ef glass pillar insulating one of these.

hg electrode leading to electrometer an accessible position G, outside the case, & then to a delicate electrometer.

kl diaphragm of metal, to prevent the air in the lowest part of the case from mixing with that above.

m an open disk of pumice stone impregnated with sulphuric acid, to keep the air in the lowest part of the case dry.

The upper disc, zz, is hung by a hook, and link n, from the bottom of a vertical stem of metal, and is steadied in a position truly parallel to the lower disc by means of three screws, o, p, q, which screw down through a plate rs rigidly connected with the vertical stem.

The vertical stem slides up and down through a tube, and is prevented from going lower than is desired, by a nut CC, screwing on its upper end.

tu is a Daniell’s battery of two good cells.

wx is a long coil of fine silk-covered wire (200 yards of No. 40 weighing, in copper alone, ½ grain per foot)

DD is a brass bar on which a metal piece E slides, so that it may be brought into contact at any moment, with any one of 101 metal studs, which are connected with the two ends of the fine coil and with 99 points dividing it into 100 equal parts. The zero stud, O, is kept connected with the cases of the condenser and electrometer, which are both of metal.

DD has a gutta percha covered wire, DF, attached to it which I hold in my hand. I thus apply at pleasure its bare end to the outer electrode G.

If in the first place E is brought to O, and then, when the upper plate of the condenser is down within 1/20 of an inch of the lower I touch the outer electrode G, with F, and next remove F, and lastly raise the upper plate of the condenser, the electrometer shows positive if the upper plate is zinc or negative if it is copper & the lower zinc.

If again the upper plate is pushed down, & the same experiment repeated, with E brought to stud No. 90, or 100, the effect of raising the upper plate at the end, is reversed (the upper plate being supposed to be zinc and the positive end of the coil being that marked +.) In the course of this experiment, there must be a sufficient pause with F touching G to allow the spot of light by which the electrometer indicates, to come to rest. The deviation of its position of rest in this case from that which it took when E was at O, is the electrometer’s measure of the static force (or difference of potentials) between studs No. 0 & No 90 or 100 as the case may be.

I go on by trial, moving E from one stud to another & always, (to check any possible mistake note the static position to which the spot of light is brought, while F remains on G. I then find to what stud E must be brought, so that the effect of lifting the upper plate of the condenser may be nothing. It is most frequently between 60 & 70 that the stud giving this neutral effect is found.

Now the static difference between 0 and 100 is not sensibly less than the full electromotive force (or intensity) of the two Daniell’s cells. Hence from .6 to .7 of this force applied between the zinc & copper discs, positive to copper & negative to zinc, does away with the Volta-electrification Zinc positive & copper negative, which metallic contact between them produces.

But I find great discrepances [sic] in the results of different trials. Sometimes the zero is obtained by putting E as low as 10, sometimes I must put it as high as 80. I generally keep a dry atmosphere in the upper part of the case, but occasionally moisten by my breath the opposed surfaces of zinc & copper. This sometimes raises the effect to 85. After leaving the apparatus for some time with a drying agent in the upper part of the case, I find the effect much lowered.

It seems probable that films of oxyde forming on the zinc plate & becoming dry after being thoroughly moistened may account for some of the variations. I am now trying from day to day as far as I can find time, to test how the effect varies & if possible learn why.

As soon as possible I shall try the experiment with other gases than atmospheric air between the liquids also with liquids containing no oxygen. It has been a most perplexing investigation so far, and is not at all in such a state as I can describe with credit, as yet. After our conversation on the subject last December, I feel sure that you will think the subject worth considering, and will excuse my writing about it although I have so little satisfactory to tell[.]

We are now settled in the College, for the first time for several years. Mrs. Thomson has gained much during the summer, and is able to walk about sufficiently not be the prisoner she would have been hitherto if living in town⁴.

She desires to be very kindly remembered to you, and with kind remembrances to Mrs. Faraday and your niece

I remain | Yours very truly | William Thomson