102 STATIC ELECTRICITY 



touching the knobs together, and the potential of each was found 

 to be 113. A had given to 13 176 and retained 113. But 176 in 

 B only produced the same potential difference as 113 in A, or the 

 capacity of B -f- the capacity of A = 173 ^-113 = 1*55. Both v 

 now discharged and B was charged afresh to potential 204. 

 charge was shared with A and each indicated a potential 118. 

 The charge which produced a potential 118 when given to A 

 produced a fall only of 204-118 = 86 when taken from B. Since 

 the capacity is inversely as the potential produced by <:i\cn 

 charge, the capacity of B -J- the capacity of A = 118-^-86=l 



Making corrections for loss of charge by " residual effect to 

 be discussed below, Faraday found that the two corrected \alucs 

 were 1*5 and 1*47, say 1*5. He assumed that the.- excess of capacity 

 for the B condenser was only half what it would have been had tin- 

 whole space been filled with shellac, and thus In- found that the 

 capacity of B was twice that of A, or the ^jxcilic induct in- 

 capacity of shellac was 2. He pointed out that this was an limit r- 

 estimate, for the hemispherical cup did not change (juitr halt' the 

 capacity of the jar, since the rod passing through the m-ck had 

 some capacity, which was the same in both conditions <>t B. With 

 a flint-glass cup in place of the shellac he found K = 1'7(>, and 

 with sulphur K = 2'24. 



When liquids were introduced into the condenser no certain 

 measurements could be made owing to conduction, and \\hcn 

 different gases replaced air no difference could he detected, for tin- 

 apparatus used was not sufficiently sen-iti\e. The difficulties <>t 

 experiment with liquids and gases ha\e only been OUTCUIMC ^ince 

 Faraday's time. 



Faraday also uaed two cowlenierB consisting of three parallel 

 equal circular plates, the middle plate formi: fkdenter With 



each side plate.* He charged the middle plate, and then slum ed that 

 by introducing a slab of shellac or sulphur between the middle 

 and one side plate the capacity on that side was increased. He 

 saw that this arrangement might be used for exact measuren 

 as indeed it has been used later in a modified form. 



Though our knowledge of the existence of specific inductive 

 capacity is entirely due to Faraday, it is not a little remarkable 

 that it was discovered by Cavendish some time between 1771 and 

 1781. But he communicated his results to no one, and t 

 remained buried in his MS. notes till these were edited anil 

 published by Maxwell in 1879 as The Electrical Researches of 

 the Hon. Henry Cavendish. His discovery was thus entirely 

 without influence on the progress of electrical knowledge. 



For many years after the publication of Faraday's original 

 paper little experimental work was done on the subject, but the 

 publication of Maxwell's Electro-magnetic Theory of Light,| 



* Ex$. Res. i. p. 413. 



t Phil Trans. , 1865, p. 459 ; Electricity and Magnetism, vol. ii. chap. xx. (1873). 



