1878.] 



Contact Theory of Voltaic Action. 



229 



employed. Contrary to our expectations, however, there appeared to 

 be no further decomposition of the india-rubber by heat, and results 

 analogous to those previously obtained with the paraffin wax were met 

 with. Probably, therefore, all the sulphur that could easily be 

 liberated had been expelled by the previous heating. At about 90° C, 

 the conductivity was sufficient to enable us to measure, with some 

 accuracy, the electromotive force, which we found to be 0"168 volt; 

 but somewhat as in the case of the gutta-percha, between 90° and 

 100° C, the electromotive force increased to 0'356 volt. This 

 increase did not appear to be as sudden as in the case of gutta-percha, 

 but, as before, the electromotive force was fairly constant on each side 

 of this place of changing. 



Shell-Lac. 



An extremely thin sheet of shell-lac was placed between plates of 

 zinc and copper, as in the preceding experiments. At low tempera- 

 tures there was evidently an electromotive force, which, at 28° C, we 

 were able to measure, as the conductivity was sufficiently great. 

 From this temperature upwards there was practically no change. In 

 fact, the electromotive force was constant and equal to 1*12 volts. At 

 temperatures higher than 6-5° C, the arrangement established its 

 charge after short-circuiting with the rapidity of an ordinary liquid 

 voltaic cell. 



Resistances of the above Electrolytes. 



It has been usual for telegraph engineers to measure the resistance 

 of gutta-percha, india-rubber, &c, after a current has been passing 

 through the material for one or for not more than three minutes. 

 Now, although this may be a very valuable test of a telegraph cable, 

 since the amount of apparent loss, through want of insulation during 

 the first small period of charge, is one of the elements affecting the 

 received current, still since this loss is due to two causes, soaking in 

 and true conductivity, and since the former is at first so very great 

 compared with the latter, the real resistance can only be ascertained 

 after the current has been on for a length of time. The practical 

 time of charging necessary to enable the current to be a true measure 

 of the conductivity seems to be less at higher temperatures, and in our 

 experiments we have endeavoured to approximate to the elimination of 

 soaking in. 



Paraffin-Wax. 



In November, 1875, we made a great number of time observations 

 of the difference of potentials of two copper plates immersed in paraffin- 

 wax, and kept asunder by three small pieces of glass, 0"191 centim. 

 thick, the upper plate having been originally charged to a certain 

 potential, and the lower plate kept in connexion with the earth. We 



