22 Introduction 



Cavendish, who explained the fact that the capacity of a glass plate 

 is greater than that of an air plate, by supposing that the electricity is 

 free to move within certain portions of the glass, supposed that when the 

 plate was more strongly electrified the electricity would be able to pene- 

 trate further into the glass, and that therefore its charge would be greater 

 in proportion to that of a simple conductor or a plate of air the stronger 

 the degree of electrification. 



But according to the experiments he made to answer this question* 

 a coated plate and a simple conductor whose charges were equal for the 

 usual degree of electrification remained sensibly equal for higher and lower 

 degrees, and if, as appeared probable from the experiments on the spreading 

 of electricity at the edge of the coating, this spreading extended further 

 for high degrees of electrification than for low, it would be necessary to 

 admit that the charge of a glass plate became less in proportion to that 

 of a simple conductor as the degree of electrification increased. Cavendish, 

 however, concluded that the experiments were hardly accurate enough to 

 warrant the deduction from them of so improbable a conclusion. 



He also found that the result of the comparison of a coated plate and 

 a simple conductor was the same whether they were charged positively 

 or negatively. 



He tried whether the capacity of a plate of rosin altered with the 

 temperature, but he could not find that it didf. In glass he found that 

 the capacity increased as the temperature rose, but the most decided 

 increase did not occur till the glass began to conduct somewhat freely. 

 Cavendish therefore does not consider the experiment quite decisive J. 



He found that the apparent capacity of a Florence flask was greater 

 when it continued charged a good while than when it was charged and 

 discharged immediately, and he found that the same was the case with 

 a coated globe of glass. This phenomenon, which Faraday called "electric 

 .absorption," has recently been carefully studied in different kinds of glass 

 by Dr Hopkinsonj(. It is connected with the long-known phenomenon of 

 the "residual charge," and the existence of such phenomena in many 

 dielectrics renders it difficult to obtain consistent values of their inductive 

 capacities; for the more rapidly the charging and discharging is effected 

 the lower is the apparent value of the capacity. It is for this reason that 

 condensers of glass cannot be used as standards of capacity when accurate 

 measurements are desired. 



Franklin had shown^f that the charge of a glass condenser resides in 

 the glass and not in the coatings, for when the coatings were removed 

 they were found to be without charge, and when new coatings were put 

 in their place the condenser thus reconstructed was found to be charged. 



* Arts. 355-3 6 5- t Art - 5 2 3- + Art. 366. 



Art. 523. || Phil. Trans. 1877, p. 599. 



H Franklin's Works, ed. Sparks, vol. v. p. 201. 



