AND MODEltN PHYSICS. 211 



is oscillatory. It does not consist of a single spark, 

 but of a series of sparks, which pass and repass in 

 opposite directions, until the energy of the original 

 charge is radiated into space or dissipated as heat; 

 the plates are then recharged and the process repeated. 

 In Hertz's experiments the oscillator was charged by 

 being connected to the secondary terminals of an 

 induction coil. 



In 1883 Professor Fitzgerald had called attention 

 to this method of producing electric waves in air, and 

 had given two metres as the minimum wave length 

 which might be attained. In 1870 Herr von l>ezo!d 

 had actually made observations on the propagation 

 and reflection of electrical oscillations, but his work, 

 published as a preliminary communication, had at- 

 tracted little notice. Hertz was the tirst to undertake 

 in 1887 in a systematic manner the investigation of 

 the electric waves in air which proceed from such an 

 oscillator with a view to testing various theories of 

 electro-magnetic action. 



It remained, however, necessary to devise an 

 apparatus for detecting the waves. When the waves 

 are incident on a conductor, electric surgings are set 

 up in the conductor, and may, under proper conditions, 

 be observed as tiny sparks. Hertz used as his de- 

 tector a loop of wire, the ends of which terminated in 

 two small brass balls. The wire was bent so that the 

 balls were very close together, and the sparks could 

 bo seen passing across the tiny air gap which separated 

 them. Such a wire will have a definite period of its 

 own for oscillations of electricity with which it may 

 be charged, and if the frequency of the electric waves 



