594 Removal of Voltaic Pole) dial- Difference. 



of exceedingly high resistance with an electromotive force of 

 about '7 volt at first, falling after heating to about one-sixth 

 of that amount. It may thus be noted that this electromotive 

 force is greatest at the time when its existence is seen to have 

 little effect on the point in question. In order, however, to 

 finally obviate any conduction effect of this kind, the oil was 

 removed and the plates carefully wiped with cotton-wool till 

 nothing but a mere film of oil remained on their surfaces. 



In the first experiment (iSTo. 1) the volta effect was still 

 absent when this had been done, but after one and a half hours, 

 on testing again, I found it had returned and deflexions of 

 about 30 were obtained in tbe same direction as in air. I 

 concluded that moisture from the atmosphere had found 

 opportunity to reach the zinc surface in some quantity. In 

 a repetition (No. 2) of the whole experiment the same effects 

 were observed up till the removal of the oil, after which 

 in this case, the return of the volta effect did not take place 

 in four days, or if at all present, it was very slightly reversed. 

 In this experiment the oil had been kept at a high tem- 

 perature for several hours and probably formed a more 

 permanent protective film on the metallic surfaces. In a 

 further experiment (No. 3) in which the high temperature 

 was maintained only for a short period as in No. 1, the results 

 again corroborated No. 1 only that the volta effect appeared 

 still more quickly. In nine days after removal of the oil 

 (except such oil films as adhered to the plates) it had 

 regained almost its pristine value, the average deflexion 

 being then about 130. 



At the conclusion of each of the three experiments, the 

 zinc plate,, on examination, appeared almost unaltered in 

 appearance — only very slightly tarnished. On the copper 

 there appeared to be a very thin transparent varnish-like 

 film. On re-eleaning the plates with glass-paper the usual 

 volta effect on air was observed. 



To sum up, I conclude that all the effects observed fall in 

 satisfactorily with the condensed electrolyte-film theory as 

 described above. On first immersion in cold oil the films 

 continued to act in their usual way, as if in air, though with 

 a somewhat less deflexion, probably owing to a minute con- 

 ductivity in the oil somewhat reducing the difference of 

 potential during the slow separation of the plates after 

 metallic contact. When heated to above the boiling-point of 

 water, the films evaporated and left nothing to effectively act 

 electrolytically on the plates. After removal of the oil 

 (unless a protective varnish-like film had been formed) the 

 moisture of the air again found access to the surface and a 

 difference of potential was again observed. 



