The Followers of Maxwell. 359 



As in the jar discharge,* the electricity surges from one 

 sheet to the other, with a period proportional to (CL)l, where 

 denotes the electrostatic capacity of the system formed by 

 the two sheets, and L denotes the self-induction of the 

 connexion. The capacity and induction should be made as 

 small as possible in order to make the period small. The 

 detector used by Hertz was that already described, namely, 

 a wire bent into an incompletely closed curve, and of such 

 dimensions that its free period of oscillation was the same 

 as that of the primary oscillation, so that resonance might take 

 place. 



Towards the end of the year 1887, when studying the sparks 

 induced in the resonating circuit by the primary disturbance, 

 Hertz noticedf that the phenomena were distinctly modified 

 when a large mass of an insulating substance was brought 

 into the neighbourhood of the apparatus ; thus confirming the 

 principle that the changing electric polarization which is pro- 

 duced when an alternating electric force acts on a dielectric 

 is capable of displaying electromagnetic effects. 



Early in the following year (1888) Hertz determined to 

 verify Maxwell's theory directly by showing that electro- 

 magnetic actions are propagated in air with a finite velocity .{ 

 For this purpose he transmitted the disturbance from the 

 primary oscillator by two different paths, viz., through the air 

 and along a wire ; and having exposed the detector to the joint 

 influence of the two partial disturbances, he observed inter- 

 ference between them. In this way he found the ratio of the 

 velocity of electric waves in air to their velocity when conducted 

 by wires ; and the latter velocity he determined by observing 

 the distance between the nodes of stationary waves in the wire, 

 and calculating the period of the primary oscillation. The 

 velocity of propagation of electric disturbances in air was in 



* Cf. p. 253. 



t Ann. d. Phys. xxxiv, p. 373. Electric Waves (English edition), p. 95. 



J Ann. d. Phys. xxxiv (1888), p. 551. Electric Waves (English edition) p. 107. 



