LIGHT AND ELECTRICITY. 135 



At flrst the gap prevents a discharge from the conductors; the air in 

 it phiys the role of insuhitor and maintains our penduhim in a position 

 diverted from tluit of e([nilibrinm. 



But when the difference of potential becomes great enough, a si)ark 

 will jump across. If the self-induction is great enough and the capacity 

 and resistance small enough, there will be an oscillatory discharge 

 whose period can be brought down to a hundred millionth of a second. 

 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 imi)ulse 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 thou- 

 sand million times per second. Thanks to this extreme frequencj^, 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 round- 

 about 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 i)hases of the vibration the difference of poten- 

 tial of the two spheres will be just great enough to cause the spai'ks 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 exciter at a 

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

 ferences can be iiroduced by redection. In the first case it will be 

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

 in the opi^osite 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 iu 

 which the spark of the resonator will be extinguished. 



Experiments with a long wire are the easier and furnish much valu- 

 able information, but they can not furnish an experimentum 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 tlmt 

 the velocity of propagation of induction across air is finite, but also 

 that it is equal to the velocity of the wave propagated along a wire, 

 conformably to the ideas of Maxwell. 



