William Snow Harris to Faraday   20 May 1855

Plymouth 20 May 1855

My dear Faraday

You have been always so generous & kind to me upon Philosophical Subjects, that I feel ashamed at offering any large apology for this further intrusion on your valuable moments. I am however desirous to satisfy my mind upon one or two points in which I am now deeply interested, will you then permit me to submit the following points to you.

First - let me ask whether you consider the following expt. as a fair illustration of the evolution of Electricity during Chemical action

- Expt:- put some coarse grains of the impure Zinc of commerce into a glass bottle A pour on them dilute sulphuric acid the water will decompose and hydrogen will escape at h. If during the effervescence a gold Leaf Electrometer be applied to the glass vessel A its leaves will diverge freely.

Well then here is clearly a development of common Electricity during chemical action.

- Now if I clearly understand you in your views of current force - in the pile - Pure zinc or amalgamated zinc is not acted on in this way the water will not be decomposed although there may arise a large Electrical Tension between the metal and the fluid - directly a metal such as copper is put into the liquid & made to touch the Zinc then Chemical Action ensues and we have a current.

Now I want to satisfy my mind as to what takes place when we use Iron or impure zinc here we have at once the same result as is brought about by the introduction of the copper plate - in the impure zinc I suppose it is the presence of other metals which brings about the result - but how of Iron - say pure Iron will not filings of pure Iron cause chemical action in dilute acid without the presence of another metal?

With respect to what is called the “Contact Theory” I have carefully read through the subject and I am obliged to conclude - that it is perfectly untenable - the Phenomena of the Pile in all their generality upon Voltas simple view of the source of power are quite impossible.

Finally I would ask - whether it may not appear to you upon further reflection, that some confusion & misapprehension arises in the application of the terms positive & negative to the metals exhibiting Electrical disturbance after contact - say I bring a plate of Zinc to touch a plate of Copper and I find that Electricity has passed or is supposed to have passed from the Copper upon the Zinc. Surely in this case the Copper should be called the positive metal & the Zinc the negative - whereas Zinc is called positive now it is a cardinal point in your philosophy that the surface from which Electricity flows is to be considered positive - your anode for example is opposite the positive Electrode - positive because the current flows from it - reciprocally for the Cathode[.]

In ordinary Electricity we consider the prime Conductor positive in the Glass Machine because it gives off Electricity & the conductor of the rubber negative because it takes up Electricity from the Prime Conductor. Now this is precisely what by the Contact Theory the Copper does in relation to the Zinc it appears to me, that the views of pos & neg as originally expressed in the contact experiments should be reversed. I remain My dear Faraday

Most truly & sincerely yours | W. Snow Harris

I suppose in the Paper in the Trans for 1801 by Wollaston¹ page 427² - In which he says “If a piece of zinc and a piece of Silver have each an extremity immersed in dilute acid &c &c &c - the Zinc is dissolved &c .[“] he does not mean pure zinc but the Zinc of commerce[.]