LIGHT AND ELECTRICITY. 133 



the borders of their province. That province, so defined, no experi- 

 mental confirmation of Maxwell's theory invaded for twenty-five years. 



What was wanted was some issue between the two theories not too 

 delicate for our coarse methods of observation to decide. There was 

 but one line of research along- which any experimentum crucis was to 

 1)(^ met with. 



The old electro-dynamics makes electro-magnetic induction take 

 place instantaneously; but according to Maxwell's doctrine it propa- 

 gates itself with the velocity of light. 



The point was then to measure, or at least to make certain, a velocity 

 of propagation of inductive effects. This is what the illustrious Ger- 

 man iihysicist Hertz has done by the method of interferences. 



The method is well known in its application to optical i>henomena. 

 Two luminous rays from one identical center interfere when they reach 

 the same point after pursuing paths of different lengths. If the differ- 

 ence is one, two, or any whole number of wave lengths, the two lights 

 reenforce one another so that if their intensities are equal, that of their 

 combination is four times as great. But if the difference is an odd 

 number of half wave lengths, the two lights extinguish one another. 



Luminiferoiis waves are not peculiar in showing this phenomenon; it 

 belongs to every periodic change which is propagated with definite 

 velocity. Sound interferes just as light does, and so must electro- 

 dynamic induction if it is strictly periodic and has a definite velocity 

 of propagation. But if the proi)agation is instantaneous there can be 

 no interference, since in that case there is no finite wave length. 



The phenomenon, however, qould not be observed were the wave 

 length greater than the distance within which induction is sensible. 

 It is therefore requisite to make the period of alternation as short as 

 possible. 



ELECTRICAL EXCITERS. 



We can obtain such currents by means of an apparatus which con- 

 stitutes a veritable electrical pendulum. Let two conductors be united 

 by a wire. If they have not the same electric potential the electrical 

 equilibrium is disturbed and tends to restore itself, just as the molar 

 equilibrium is disturbed Avhen a pendulum is carried away from the 

 position of repose. 



A current is set up in the wire, tending to equalize the potential, just 

 as the pendulum begins to move so as to be carried back to the position 

 of repose. But the pendulum does not stop when it reaches that posi- 

 tion. Its inertia carries it farther. Nor, when the two electrical con- 

 ductors reach the same potential, does the current in the wire cease. 

 The equilibrium instantaneously existing is at. once destroyed by a 

 cause analogous to inertia, namely self-induction. We know that when 

 a current is interrupted it gives rise in parallel wires to an induced cur- 

 rent in the same direction. The same effect is produced in the circuit 



