228 



Profs. W. E. Ayrton and John Perry. [Mar. 21, 



As in the case of paraffin, the conductivity calculated from the 

 time readings, whilst the zinc and copper plates were establishing 

 their charge, is greater than the conductivity measured independently 

 by two copper plates charged with a battery for about five minutes 

 and then kept insulated. 



Ebonite. 



Between the zinc and copper plates was placed an ebonite sheet, 

 having an average thickness of 0'35 millim. It was chemically clean, 

 and there was no surface conduction at even low temperatures. We 

 found, as in the previous cases, that there was evidence of an electro- 

 motive force, but the charging proceeded very slowly until about 

 36° C, when the conductivity had greatly increased. During thirty- 

 five minutes the temperature steadily increased to 74° C, and as there 

 was no indication of a maximum charge, we let it increase without 

 short-circuiting during this thirty-five minutes, at the end of which 

 time the deflection indicated an electromotive force of 3 volts. After 

 short-circuiting the charge again established itself, but it did not 

 increase as rapidly as it had done at first at the lower temperature, 

 although from our independent experiments we know that the con- 

 ductivity is greater at higher temperatures. Precautions have been 

 taken to avoid any sliding of the ebonite plates during the experi- 

 ments ; for certain preliminary observations showed that such sliding 

 would produce charges of electricity which would mask mere voltaic 

 results. From the mathematical expressions for conductivity and 

 from our experimental determinations of the conductivity at any given 

 temperature, we endeavoured, by calculation, to discover whether the 

 first set of time readings indicated an approach to a maximum, but we 

 obtained inconsistent results ; the maximum electromotive force thus 

 calculated varying from 3 to 12 volts. After the experiments the 

 zinc plate was found to be only slightly dulled in its polish and not 

 coloured, neither did the copper plate appear much changed in appear- 

 ance, but a lead weight which had been laid on the zinc to keep it in 

 close contact with the ebonite was found to be quite red. 



It is evident, therefore, that this arrangement had nothing in 

 common with an ordinary simple cell, the great charging of the plates 

 - being due to a decomposition of the ebonite by heat. 



India-Rubber. 



In November, 1875, we endeavoured to find the electromotive force 

 of a simple cell formed by placing a sheet of india-rubber containing 

 sulphur between zinc and copper plates, and we then obtained results 

 analogous to those which have just been described for ebonite. During 

 the present investigation, we expected to obtain the same results, since 

 we used the piece of india-rubber which had previously been 



