ELECTRICAL APPARATUS. 161 



Specific Inductive Capacity. This property has since been inves- 

 tigated by various experimenters, and notably by Boltzmann. 

 Within the last twelve months it has been shown by Kerr, of 

 Glasgow, that the property of optical double-refraction is developed 

 in insulating solids and liquids when a difference of electrical 

 potential is maintained between opposite surfaces. Another 

 important matter connected with the properties of insulating 

 media under these circumstances is the difference of potential 

 required to make a discharge of electricity take place through a 

 layer of given thickness. This has been investigated as yet 

 chiefly by Riess, Rijke, and'Sir William Thomson. 



The phenomena accompanying the discharge of accumulated 

 electricity, and the effects due to it, belong strictly to the subject 

 of electrodynamics. The following may be mentioned as some 

 of the chief points that have been investigated in connection 

 with the discharge : the appearance presented by it in air or in 

 other gaseous media at various pressures, when viewed either 

 with the naked eye or through the spectroscope; the duration 

 of the electric spark ; its heating action ; its oscillatory character ; 

 and its mechanical effects. 



II. ELECTRODYNAMICS.* 



If two insulated conductors A and B, at different electrical poten- 

 tials, are connected by another conductor, they very quickly 

 assume a condition of electrical equilibrium characterised by uni- 

 formity of potential throughout the system formed by them and 

 the connecting conductor. During the short period occupied by 

 the process of electrical equalisation, the connecting conductor 

 exhibits special properties, not possessed by it under other circum- 

 stances, which are usually summed up in the statement that it is 



* By many writers, especially on the Continent, this term is restricted tc- 

 the part of the subject which deals with the mutual force acting between 

 conductors traversed by electric currents ; it is here used to include the whole 

 of the branch of electrical science which deals with the effects of electricity 

 in motion. 



y 



