328 CLASSICS OF MODERN SCIENCE 

 The oscillatory discharge would not, it is true, last long by itself ; but 

 it is kept up by the Ruhmkorff coil, whose current is itself oscillatory 

 with a period of about a hundred-thousandth of a second, and thus 

 the pendulum gets a new impulse as often as that. 



The instrument just described is called a resonance exciter. It pro- 

 duces oscillations which are reversed from a hundred million to a 

 thousand million times per second. Thanks to this extreme frequency, 

 they can produce inductive effects at great distances. To make these 

 effects sensible another electric pendulum is used, called a resonator. 

 In this the coil is suppressed. It consists simply of two little metallic 

 spheres very near to one another, with a long wire connecting them in 

 a roundabout way. 



The induction due to the exciter will set the resonator in vibration 

 the more intensely the more nearly the natural periods of vibration are 

 the same. At certain phases of the vibration the difference of poten- 

 tial of the two spheres will be just great enough to cause the sparks to 

 leap across. 



PRODUCTION OF THE INTERFERENCES 



Thus we have an instrument which reveals the inductive waves 

 which radiate from the exciter. We can study them in two ways. 

 We may either expose the resonator to the direct induction of the ex- 

 citer at a great distance, or else make this induction act at a small dis- 

 tance on a long conducting wire which the electric wave will follow 

 and which in its turn will act at a small distance on the resonator. 



Whether the wave is propagated along a wire or across the air, in- 

 terferences can be produced by reflection. In the first case it will be 

 reflected at the extremity of the wire, which it will again pass through 

 in the opposite direction. In the second case it can be reflected on a 

 metallic leaf which will act as a mirror. In either case the reflected 

 ray will interfere with the direct ray, and positions will be found in 

 which the spark of the resonator will be extinguished. 



Experiments with a long wire are the easier and furnish much 

 valuable information, but they cannot furnish an expcrimentum crucis, 

 since in the old theory, as in the new, the velocity of the electric wave 

 in a wire should be equal to that of light. But experiments on direct 

 induction at great distances are decisive. They not only show that 



