158 



Dr. J. Hopkinson. 



[Nov. 17, 



my previous experiments* was used. Advantage was taken of the 

 fact that at the time when there is a balance the potentials of the 

 interiors of all the condensers are the same. Let the ring O of the 

 guard-ring condenser be in all cases connected to J, let the inner 

 plate of the guard-ring be connected to J as in fig. 2, and let a 

 balance be obtained. Let the inner plate be now transferred to I as 

 in fig. 3, and again let a balance be obtained ; the difference of the 



Fig. 3. 



two readings on the slide represents on a certain arbitrary scale the 

 capacity of the guard-ring condenser at its then distance. 



In some cases it was necessary to adjust both condensers to obtain 

 a balance, then the value of a movement of the scale of one con- 

 denser in terms of the other was known from previous experiment. 

 In some cases it was found most convenient to introduce a condenser 

 of capacity known in divisions of the scale of the sliding condenser 

 coupled as forming part of the condenser J. The old method of 

 adding the opposite charges of two condensers then connecting to the 

 electrometer and adjusting until the electrometer remained undis- 

 turbed was occasionally used as a check; it was found to give 

 substantially the same results as the method here described when the 

 substance insulated sufficiently well to give any results at all. 



Colza Oil. — This oil had been found not to insulate sufficiently well 

 for a test by the method of my former paper. Most sample?, however, 

 were sufficiently insulating for the present method. Seven samples 

 were tested with the following mean results : — 



* ' Phil. Trans.,' 1878, Part I. 



