Faraday to William Thomson   8 December 1859

[Royal Institution embossed letterhead] | Brighton | 8 Decr. 1859.

My dear Thomson

I have been away from home, so am able only now to read & answer your letter¹. I did not write in reply to Mr MacFarlane², for his account raised so many ideas & doubts, that I thought I would wait for more matter. You puzzle me greatly; & I am in great doubt, because I never can judge an experiment or make up my mind about it without seeing it. No description suffices to answer all the mental inquiries that arise about the conditions. As you say, you have just got hold of Voltas experiment; only you refer to the air place, where the zinc & copper oppose each other for that cause of the final effect, which he finds in the place of metallic contact.

You seem to have made out that the place of excitement is really at x and not at a. I suppose if Voltas contact were effectual your charged index which placed at x goes either towards the Z or C according it is neg or positive, ought to do the same thing if placed over a; for I do not see why the metallic contact there should undo or hide the electric state which it is supposed to bring on. How is this in experiment? Is your charged index indifferent at a & other places except at x, where there is separation?

You seem to refer the electric state at x to the mutual chemical relations of the zinc & copper;- as if the chemical relations of the air there went for nothing. How would it be, however, if the zinc & copper ring were in such different atmospheres as oxygen carbonic acid - hydrogen, &c the oxygen being ozonified if you like? Would they be indifferent as you seem to expect naphtha will be. You might perhaps select atmospheres which would act more on copper than on zinc, as Sul Hydrogen; or at all events as much or more on silver than on zinc.

If the action be in & through the air place x., and that varies with distance, as I think you shew by one experiment, then, should not two half rings of zinc approximated in different degrees at x and a shew a difference of action at the two places, & therefore a difference of state? Or if metallic contact at a is required in the first instance, suppose that made by a metallic arc & the arc then removed, what state will the two opposed surfaces at a have? and also those at x?

Suppose the contact at a made also by water & by air what are the results in comparison with those of contact by metal.

Does a piece of Zinc alone become positive in air i.e without any metallic contact with copper or other metal - I think I remember an experiment by De la Rive in support of the chemical origin of the electricity of the voltaic pile, in which, passing chlorine through a tube of platinum, he found either the issuing chlorine or the tube electric.

Supposing your charged aluminium index was over x, in figure page 2., it will go either towards the zinc or the copper, according as it has been rendered Pos or Neg. If the place x was closed the metals being there brought into contact, would the signs of electricity in the ring cease? and then on opening the place would they appear there again?

Is there no transfer of Electricity from copper to zinc at the place of metallic contact?

Is it possible that the metal of the index may act a part in the play at the place x? It is evidently within the reach of the sources of action if they exist there;- or else it would not be affected: Would indexes of zinc, Gold, Copper &c, behave differently one to another?

I dare say you have by this time answers to many of these questions & many others, that would, with me, arise in turn. But I shall wait to hear your news.

I understand your self acting condenser; but all the influencing circumstances can only be learned, as you are finding, by trial & experiment.

Kindest remembrances to Mrs. Thomson and heartiest wishes for your quick success[.] Remember that the more embarrassing the effects may be with you just now the more novel & important may be the principles involved in them. Any thing regarding the first motions of Electricity among metals & surrounding exciting bodies must be of the utmost consequence to the progress of the science.

Ever Yours | M. Faraday