326 CLASSICS OF MODERN SCIENCE 



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 when 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 in- 

 duced current in the same direction. The same effect is produced in 

 the circuit itself, if that is not broken. In other words, a current will 

 persist after the cessation of its causes, just as a moving body does not 

 stop the instant it is no longer driven forward. 



When, then, the two potentials become equal, the current will go 

 on and give the two conductors relative charges opposite to those they 

 had at first. In this case, as in that of the pendulum, the position of 

 equilibrium is passed, and a return motion is inevitable. Equilibrium, 

 again instantaneously attained, is at once again broken for the same 

 reason ; and so the oscillations pursue one another unceasingly. 



Calculation shows that the period depends on the capacity of the 

 conductors in such a way that it is only necessary to diminish that 

 capacity sufficiently (which is easily done) to have an electric pendu- 

 lum capable of producing an alternating current of extremely short 

 period. 



All that was well enough known by the theoretical researches of 

 Lord Kelvin and by the experimentation of Federson on the oscillatory 

 discharge of the Leyden jar. It was not that which constituted the 

 originality of Hertz. 



