JAMES CLERK MAXWELL 327 



But it is not enough to construct a pendulum ; 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 period 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 pendulum whose natural period is, 

 say, a hundred-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 apparatus into two symmetrical parts, which are connected to the 

 two poles of a Ruhmkorff coil. The induced current begins to charge 

 the two conductors, and the difference of their potential increases 

 with relative slowness. 



At first the gap prevents a discharge from the conductors ; the air 

 in it plays the role of insulator and maintains our pendulum in a posi- 

 tion diverted from that of equilibrium. 



But when the difference of potential becomes great enough, a spark 

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

 city and resistance small enough, there will be an oscillatory discharge 

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



