222 ELEMENTARY LESSONS ON [CHAP. iv. 



small, and the value of the expression -r^ will become 

 very great ; which proves the statement that the capacity 

 of a condenser depends upon the thinness of the layer 

 of dielectric. 



268. Specific Inductive Capacity. Cavendish 

 was the first to discover that the capacity of a condenser 

 depended not on its actual dimensions only, but upon 

 the inductive power of the material used as the dielectric 

 between the two surfaces. If two condensers (of any of 

 the forms to be described) are made of exactly the same 

 size, and in one of them the dielectric be a layer of air, 

 and in the other a layer of some other insulating sub- 

 stance, it is found that equal quantities of electricity 

 imparted to them do not produce equal differences- of - 

 potentials ; or, in other words, it is found that they have 

 not the same capacity. If the dielectric be sulphur, 

 for example, it is found that the capacity is about three 

 times as great ; for sulphur possesses a high inductive 

 power and allows the transmission across it of electro- 

 static influence three times as well as air does. The 

 name specific inductive capacity 1 was assigned by 

 Faraday to the ratio between the capacities of two con- 

 densers equal in size, one of them being an air-condenser, 

 the other filled with the specified dielectric. The 

 specific inductive capacity of dry air at the temperature 

 o C, and pressure 76 centims., is taken as the standard 

 and reckoned as unity. 



Cavendish, about the year 1775, measured the specific 

 inductive capacity of glass, bees -wax, and other sub- 

 stances, by forming them into condensers between two 

 circular metal plates, the capacity of these condensers 

 being compared with that of an air condenser (resem- 

 bling Fig. 30) and with other condensers which he 



1 The name is not a very happy one, specific inductivity would have been 

 better, and is the analogous term, for dielectrics, to the term "specific con- 

 ductivity " used for conductors. The term dielectric capacity is also used by 

 some modern writers. 



