134 LIGHT AND ELECTRICITY. 



itself, if that is not broken. In other words, a cnrrent 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 e(j[ual, 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 equi- 

 librium is i^assed, and a return motion is inevitable. Equilibrium, 

 agjiin instantaneously attnined, 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 cajiacity of the con- 

 ductors in such a way that it is only necessary to diminish that capacity 

 sufnciently (which is easily done) to have an electric pendulum 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 Federsen on the oscillatory 

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

 originality of Hertz. 



Fk;. 1.— The liertz cxoitor. 



But it is not enough to construct a i^endulum; it is further requisite 

 to set it into oscillation. For that, it is necessary to carry it off from 

 equilibrium and to let it go suddenly, that is to say, to release it in a 

 time short as compared to the i)eriod of its oscillation. 



For if, having pulled a pendulum to one side by a string, we were to 

 let go of the string more slowly than the pendulum would have 

 descended of itself, it would reach the vertical without momentum, and 

 no oscillation would be set up. 



In like manner, with an electric i)endulum whose natural period is, 

 say, a liundred-millionth of a second, no mechanical mode of release 

 would answer the purpose at all, sudden as it might seem to us with 

 our more than sluggish conceptions of promptitude. How, then, did 

 Hertz solve the problem*? 



To return to our electric pendulum, a gap of a few millimeters is 

 made in the wire which joins the two conductors. This gap divides our 

 ai^paratus into two symmetrical parts, which are connected to the two 

 poles of a Kuhmkorft" coil. The induced current begins to charge the 

 two conductors, and the difference of their potential increases with 

 relative slowness. 



