238 



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



ment, which was found to be , 000208 microfarad. The area of each 

 of the platinum plates, between which the india-rubber was placed, 

 was 45*6 square centims. The thickness of the india-rubber was 

 0*106 centim., so that the calculated specific inductive capacity was 

 4*8. This seems too large ; but an error, if there be one, in the deter- 

 mination of the capacity would have made the calculated resistance 

 too small, whereas the resistance is much higher than that hitherto 

 obtained with any of the forms of india-rubber experimented on by 

 electricians. "We found that the capacity of the electrometer quadrants 

 was neglectable in these, and in the ebonite observations. We give 



1 E 



the temperature and the corresponding values of - log — , and of x in 



t E 2 



megohms for the points S, a, b, c, T of the curve ST. 





Temperature. 



1 ! Ei 



7 log E; 



X 



in megohms. 



s 



67° C. 



o -oioo 



5391 x 10 6 



a 



75° 



-0187 



2882 x 10 6 



b 



81° -6 



-0275 



1960 x 10 6 



c 



86° 7 



-0390 



1382 x 10 6 



T 



90° 7 



-0531 



1015 x 10 6 



II. " On the jocosity of Dielectrics." No. I. By W. E. Ayrton 

 and JohN Perry, Professors in the Imperial College of En- 

 .gineerin|r, Tokio, Japan. Communicated by Professor Sir 

 W. Thomson, LL.D., F.R.S. Received October 2, 1877. 



It is well known that india-rubber has a greater specific resistance 

 and a less specific inductive capacity than gutta-percha. Now a 

 popular explanation of this might be given as follows : — Imagine a 

 portion of the india-rubber in an india-rubber condenser replaced by a 

 metallic or other conducting substance either in a single piece or in the 

 form of grains scattered throughout the mass of the dielectric, then 

 the resistance bf the condenser will be diminished while its capacity 

 will be increased. As more and more metal is introduced into the 

 dielectric the capacity will become greater and greater, so that at last, 

 if all the india-rubber be replaced by metal, we ought to consider such 

 a condenser as having an extremely great capacity, although on account 

 of the conduction it would be impossible to experimentally measure 

 the charge. If the above consideration, which of course really applies 

 to a dielectric of heterogeneous constitution, be also applicable to 

 different homogeneous dielectrics, then we might expect to find that 

 for all dielectrics the specific inductive capacity diminishes as the 

 specific resistance increases. Now the experiments we have lately 

 been engaged on concerning specific resistances enable us to prove 

 that in the case of several well-known dielectrics this is really so, as will 

 be seen from the following table : — 



