358 The Followers of Maxwell. 



Continuing his experiments, Hertz* found that a spark 

 could be induced in the open or secondary circuit even when it 

 was not in metallic connexion with the primary circuit in which 

 the electric oscillations were generated; and he rightly inter- 

 preted the phenomenon by showing that the secondary circuit 

 was of such dimensions as to make the free period of electric 

 oscillations in it nearly equal to the period of the oscillations 

 in the primary circuit ; the disturbance which passed from one 

 circuit to the other by induction would consequently be greatly 

 intensified in the secondary circuit by resonance. 



The discovery that sparks may be produced in the air-gap 

 of a secondary circuit, provided it has the dimensions proper 

 for resonance, was of great importance : for it supplied a method 

 of detecting electrical effects in air at a distance from the primary 

 disturbance ; a suitable detector was in fact all that was needed 

 in order to observe the propagation of electric waves in free 

 space, and thereby decisively test the Maxwellian theory. To 

 this work Hertz now addressed himself.f 



The radiator or primary source of the disturbances studied 

 by Hertz may be constructed of two sheets of metal in the 

 same plane, each sheet carrying a stiff wire which projects 

 towards the other sheet and terminates in a knob ; the sheets 

 are to be excited by connecting them to the terminals of an 

 induction coil. The sheets may be regarded as the two coatings 

 of a modified Leyden jar, with air as the dielectric between 

 them ; the electric field is extended throughout the air, instead 

 of being confined to the narrow space between the coatings, as 

 in the ordinary Leyden jar. Such a disposition ensures that 

 the system shall lose a large part of its energy by radiation 

 at each oscillation. 



* Loc. cit. 



t Sir Oliver Lodge was about this time independently studying electric oscilla- 

 tions in air in connexion with the theory of lightning-conductors : cf. Lodge, 

 Phil. Mag. xxvi (1888), p. 217. So long before as 1842, Joseph Henry, of 

 Washington, had noticed that the inductive effects of the Leyden jar discharge 

 could be observed at considerable distances, and had even suggested a comparison 

 with " a spark from flint and steel in the case of light." 



