[ 529 ] 



LXI. A Method for determining the Specific Inductive Capacity 

 of Dielectrics. By Frederick T. Trouton, M.A., D.Sc, 

 and W. E. Lilly, Whitic. Exhib* 



ON comparing the energy of a condenser charged with a 

 certain quantity of electricity, first as a simple air- 

 condenser, and secondly when a dielectric sheet of greater 

 specific inductive capacity is introduced between the plates, 

 we at once see that the energy in the second case is less than 

 in the first ; since the energy equals | QV, and V the differ- 

 ence in potential between the plates is less after the sheet is 

 introduced than before, Q the quantity of electricity being the 

 same in both cases. 



In consequence, if a sheet of, say, sulphur be introduced 

 between the plates of a charged condenser, it should be acted 

 on by a force tending to suck it in between the plates. In 

 short, the electric energy in the system being less after the 

 introduction of the sulphur than before, work must have been 

 done by the electric forces during the operation. 



The first experiments made were directed towards proving 

 this deduction from simple theory, and they conclusively 

 showed that such a force existed. 



The lines of force in a condenser run from one plate to the 

 other, so that here the movement takes place at right angles 

 to these lines of force. But it must be remembered that at 

 the edge of the condenser the lines of force are curved out- 

 wards, more especially if a sheet of, say, sulphur is introduced 

 there, and these, tending to contract, draw the sheet inwards. 



The amount of this force depends on the specific inductive 

 capacity of the dielectric of which the sheet is made, so 

 that by observing the force we may determine this quantity 

 for any given substance. 



It is more convenient, in making experiments for this pur- 

 pose, to keep the difference in potential of the plates constant 

 by means of some source of electromotive force such as a 

 storage battery, than to keep the charge constant ; let V be 

 this difference measured in electrostatic units. Then to find 

 the force in terms of K, the specific inductive capacity of the 

 sheet, suppose the sheet (fig. 1) to be withdrawn a certain 

 distance x : the work done equals the energy pumped back to 

 ihe battery through the consequent diminution of the charge 

 of the condenser. Let E, E 1? and E/ be the energy, per cubic 

 centimetre of the sheet, of the air between the plates, and of 

 the air between the sheet and the plates ; and if the thickness 

 of the sheet be a, and the distance apart of the plates be 

 (a + b), then, provided the sheet is long, and well in between 

 * Communicated by the Authors. 



