MR. J. E. H. GORDON ON MEASUREMENTS OF ELECTRICAL CONSTANTS. 427 
The formula of calculation. 
Let the reading of a when there is only air in the balance be a x , and that when the 
dielectric is inserted a. 2 . 
A dielectric plate of thickness b being inserted displaces a thickness b of air. A 
dielectric plate of thickness b and specific inductive capacity Iv acts like a plate of air 
of thickness^:. 
Therefore the effect of inserting the plate has been the same as if we had decreased 
the distance a b by a quantity b — 
b 
K* 
But when we have again brought the electrometer to zero by the screw we have 
increased a b by a quantity a 2 —cq, and as this increase exactly compensates the 
apparent decrease produced by the dielectric, we must have 
b — 
l 
__— a 2 a 2 , 
or 
K= 
&-(a 2 -cq) 
and this formula was used to calculate the results of the following experiments. 
The experiments on ebonite show that the results given by the formula agree with 
each other over a very wide range of values of b. 
Fig. 6. 
This formula assumes that those lines of force between a and b which pass through 
the dielectric are approximately straight lines, also that all the lines pass through it, 
and that no portion, or at least no sensible portion, of them pass round its edge. To 
test the truth of this assumption the following experiment was made:—A plate of 
ebonite, (3, fig. 6, being placed in the usual way between a and b, a was adjusted so as 
3 I 2 
