210 JAMES CLE11K MAXWELL 



L, the coefficient of self induction, and (,' the capacity 

 of the condenser. These quantities can be calculated, 

 and hence the time of an oscillation is known. From 

 such an arrangement waves radiate out into space. If 

 we could measure by any method the length of such a 

 wave we could determine its velocity by dividing the 

 wave length by the period. lUit it is clear that since 

 the velocity is comparable with that of light the wave 

 length will be enormous, unless the period is very 

 short. Thus, a wave, travelling with the velocity of 

 light, whose period was '000 i second, such as the 

 waves Schiller worked with, would have a length of 

 0001 x :K),000,000,00() or a,(JOO,000 centimetres, and 

 would be quite immeasurable. Before measurements 

 on electric waves could be made it was necessary ( f ) 

 to produce waves of sufficiently rapid period, (2) to 

 devise means to detect them. This is what Hertz did. 

 The wave length of the electrical oscillations 

 can lo reduced by reducing either the electrical 

 capacity of the system, -or the coefficient of self- 

 induction of the wire. Hertz adopted both these 

 expedients. His vibrator, in some of his more im- 

 portant experiments, consisted of two square brass 

 plates 40 cm. in the side. To each of these 1 is attached 

 a piece of copper wire about :>0 cm. in length, and each 

 wire ends in a small highly-polished brass hall. The 

 plates are placed so that the wires lie in the same 

 straight line, tin? brass balls being separated by a very 

 small air gap. The two plates are then charged, the 

 one positively the other negatively, until the insulation 

 resistance of the air gap breaks down and a discharge 

 passes across. Under these conditions the discharge 



