William Thomson to Faraday   10 March 1860

2 College, Glasgow | March 10, 1860

My dear Faraday

After long waiting I have been able to try some of the experiments we spoke of with the aid of an electrometer which makes them easy. It is one I have had made, to serve for the self recording atmospheric apparatus at Kew¹, and in the course of a week’s trials I have had of it, I have used a glass fibre much lighter than that which will give it a metallic sensibility for the work for which it is designed. In the condition in which I have used it, it has given as much as 110 divisions on each side of zero (each divn. 1/40i., on a scale 22i from the mirror) is the effect of direct & reverse applications of a single cell of Daniell’s battery. This effect it shows perfectly through my whole lecture room (holding about 100) with only a paraffine lamp. With a good mirror & the electric light, there would I am sure be no difficulty in removing the scale far enough to double the sensibility & yet good in effect visible through the whole of your theatre in the Royal institution. Thus the electroscopic effect of a single thermoelectric bism[uth]-antimony element could easily be shown.

In showing it to my class I preferred a rather less sensibility - giving about 60 divns (or an inch & a half) as the effect of a single cell of Daniell’s.

If I lay a disc of copper on the table beside the instrumen[t] & make a good metallic communication between it (the copper) & the metal case of the electrometer, and then place a zinc disc resting on it, but separated from it by three thin pieces of glass, and connect the zinc by a wire with the insulated halfring, I find a deflection shown by the spot of light gradually supervenes. If a metallic arc is applied to make a communication betw. the disc & copper the spot comes back to zero, & again gradually moves when the zinc is left supported on the pieces of glass. If pieces of paper are substituted for the glass the same thing take place but more rapidly. If the paper is wet, or if a drop of water is inserted between the zinc & copper discs so as to touch each while the glass separating pieces are retained, the samething takes place instantaneously (altho after a metallic commn. is established, it takes a little time before the full deflection is again reached.) I believe when properly tried the final amount of the deflection will be the same in each case, although I have never got it quite so much with the glass or the paper, as with the drop of water. In fact the glass if perfectly dry would not in years conduct, by proper conductivity through its own substance, as much as passes in a few minutes or even when in its ordinary moist condn. in the atmosphere: and the pieces between the discs only differ from the glass supports of the insulated halfring in being in this moist state instead of in an artificially dry atmosphere. I think I can now answer with certainty your question as to the condition of perfectly unelectrified zinc & copper approaching before contact². They will exercise no force whatever on an electrified body in their neighbourhood; but the moment a metallic arc is placed connecting them, such a body, if free will move from the zinc or from the copper, according as it is positively or negatively electrified itself. I hope to show a single experimental demonstration of this; but in the mean time I infer it from a combination of what I have seen.

The two half rings of my electrometer are of one metal (brass.) If large discs of copper and zinc are connected respectively with the two half rings (one of which I always chief in metallic communication with the case,) and of these discs of zinc & copper are first well discharged at a distance & then (one of them being next well insulated) if they are gradually approximated to one another, the spot of light gradually moves so as to show zinc negative. When they are within a short distance, say 1/10 of an inch, the deflection does not increase sensibly on their being brought closer & closer; but when they are brought into contact, it comes to its zero position suddenly. The maximum reflection here is I believe exactly the same as the true zinc copper single fused element. Again, if I substitute for the two half rings of the electrometer a compound ring of zinc & copper in the places of the two halves respectively soldered together, and if I give necessarily equal measured positive & negative charges to the index, the spot of the light moves to one side & the other alternately so as always to show the zinc pos. to the copper neg. The amount of the deflection I believe will be found to be the same as that of the single zinc copper water element, tested with the electrometer [word illegible] ordinary iron & needle always charged to the same degree[.] Hence I infer that if the zinc half ring was connected with the zinc disc in the previous expt. & if a true discharge of the half ring were made before beginning, the spot of light would not stir, on the two discs being approximated. At the instant of the two discs touching one another, it would move so as to show the deflection demonstrated by the second expt.

I am quite ashamed when I look through what I have written, and finding the appearance irremediably bad, I have only endeavoured as far as possible to make it legible. I feel reluctant to trouble you with it, but I send it as you kindly expressed an interest in some of what I was doing before on the subject.

Mrs. Thomson is feeling much better than she has done for a long time. She sends her kindest remembrances. When I saw Mr. Crawford last a few days ago I told him that I was going to write to you and he asked me to say that his family are all well & that he hopes you are so also.

Believe me Yours most truly | William Thomson