1889.] 



Specific Inductive Capacity of Dielectrics. 



295 



40 in electrostatic units ; the formula S/4tt£ would, where 

 S — 7T x 15 2 and t = 2, give 28, so that of the 40 units of capacity, 28 

 are due to the two disks and 12 to the presence of the other con- 

 ductors. This was verified by determining by the tuning-fork method 

 the capacity of the condenser when the distance between the disks had 

 a series of values. 



When the distance between the disks was 2 cm., the mean of several 

 determinations of the wave-length along the wire was 8*25 metres. 

 The value calculated by the formula 27r\/(LC), where C = 40 and 



(8Z \ 

 log — — 2 j, where I = length of circuit (supposed circular) 



= 50 cm., and d the diameter of the wire = - 3 cm., is 8 metres. 

 When the plates were separated by pieces of plate glass 2 cm. thick, 

 the wave-length was 11"75 metres. Thus, if K is the specific inductive 

 capacity of the glass, 



11 75 /28K + 12 



8^25" ~ V 40 ' 



K = 2 7, and yK = 1 '65. 



The determination of the specific inductive capacity of the glass by 

 the tuning-fork method was difficult, owing to electric absorption; 

 the values for K obtained in this way varied between 9 and 11. We 

 see, therefore, that for vibrations whose frequency is 3 X 10 10 /11'75 X 10 3 , 

 or 25,000,000 per second, the specific inductive capacity is very nearly 

 equal to the square of the refractive index, and is very much less 

 than the value for slow rates of reversals. The discrepancy is pro- 

 bably due to the cause which produces the phenomenon of anomalous 

 dispersion in some substances, and indicates the existence of molecular 

 vibrations having a period slower than 25. 000,000 per second. The 

 behaviour of the glass under electrical oscillations of the critical 

 period would form a very interesting subject of investigation. 



The specific inductive capacity of ebonite was determined in a 

 similar way; the wave-lengths, when the plates were separated by 

 air and ebonite respectively, were 8*5 metres and 1075 metres, giving 

 as the specific inductive capacity of ebonite 1*9. The value determined 

 by the tuning-fork method was 21. 



The specific inductive capacity of a plate made of melted stick 

 sulphur was also tried : the wave-length without the sulphur was 8*25, 

 with it 11 "5, giving as the specific inductive capacity of sulphur 2'4. 

 The value determined by the tuning-fork method was 2*27. Thus, 

 for ebonite and sulphur the values determined by the two methods 

 agree as well as could be expected, while for glass the results are 

 altogether different. 



