224 ELEMENTARY LESSONS ON [CHAP. iv. 



tact. If the capacity of one was greater than the other 

 the final potential would not be exactly half the original 

 potential, because they would not share the charge 

 equally, but in proportion to their capacities. The 

 potentials of the charges were measured before and 

 after contact by means of a torsion balance. * Faraday's 

 results showed the following values: Sulphur, 2 '26; 

 shellac, 2*0; glass, 1-76 or more. 



27O. Recent Researches. Since 1870 large addi- 

 tions to our knowledge of this subject have been made. 

 Gibson and Barclay measured the inductive capacity of 

 paraffin by comparing the capacity of an air condenser 

 with one of paraffin by means of a sliding condenser, and 

 a divided condenser called a " platymeter," using a 

 quadrant electrometer as a sensitive electroscope to 

 adjust the capacity of the condensers exactly to equality. 

 Wiillner, Boltzmann, and others, have also examined 

 the inductive capacity of solid bodies by several methods. 

 Hopkinson has examined that of glass of various kinds, 

 using a constant battery to produce the required differ- 

 ence of potentials, and a condenser provided with a 

 guard -ring for a purpose similar to that of the guard- 

 ring in absolute electrometers. Gordon has still more 

 recently made a large number of observations, using a 

 delicate apparatus known as a statical " induction 

 balance," which is a complicated condenser, so arranged 

 in connection with a quadrant electrometer that when 

 the capacities of the separate parts are adjusted to 

 equality there shall be no deflection in the electrometer, 

 whatever be the amount or sign of the actual electrifi- 



1 The value of the specific inductive capacity k could then be calculated 

 as follows : 



Q = VC = V'C + V'Ck 



(where C is the capacity of the first apparatus and V its potential, and V 

 the potential after communication with the second apparatus, whose 

 capacity is Gfc) : 



hence V = V (i -f k) 



