STATIC ELECTRICITY 



In ordinary methods of determining K by experiments on capa- 

 city, we charge and discharge a condenser in times enormously long 

 compared with the period of a visible light vibration, and we should 

 only, therefore, expect to find v/K = yu if the value of M is that for 

 very long waves with period long, and if that value is unaffected 

 by absorption that is, if the substance is transparent to very 

 long waves. 



We shall now give an account of some of the determinations of 

 the dielectric constant for solids, liquids, and ga^-. selecting typical 

 methods, and not attempting to give any complete account of the 

 subject.* 



At the time when Maxwell published his treatise on Electricity 

 and Magnetism the only substance of which the dielectric constant 

 was at all accurately known was paraffin. (iib-on and Kuvlay 

 (Phil. Trans., 1871,* p. 573) had shortly betore found K to he 

 1'975. From the refractive index of melted paraffin for the A. 1 ). 

 and H lines Maxwell calculated the refractive index for light of 

 infinite wave-length to be T42 ., whence /z 2 = '' 



Boltzmann's condenser method for solids.f Holt/m ann 

 used an air condenser (Fig. 84) with parallel plates so arranged 



FIG. 84. 



that a slab of the dielectric to be experimented on could be inserted. 

 He compared the capacities of the condenser with and without the 

 slab by charging it in each case to a definite potential, and then 

 sharing the charge with a quadrant electrometer with a small air 

 condenser added in order to increase the capacitv. This was done 

 so that any change in the capacity of the electrometer due to the 

 motion of the needle should be negligible. The fall of potential din- 

 to the division of the charge gave the ratio of the capacity of the 



* An account of the earlier work on specific inductive capacity will be found 

 in Gordon's Electricity and Magnetum, vol. i. chap. xi. ; or Gray's Ab* 

 sinrmcnti*, vo 1 . i. Later work is described in Winklemann's Handbnch, vol. iv. 



f Carl's Repcrtorinin. x. p. 1<>9. See Gordon's Electricity, vo'. i. 



