254 ELEMENTS OF ELECTRICITY AND MAGNETISM. 



versed distortion of the chains of ether cells is then relieved by a 

 reversed slip (a reversed current in the rods and along spark), 

 and the above described action is repeated over and over again 

 until the original energy of the electrical field has been dissipated. 

 The oscillatory changes above described take place so rapidly 

 that the portions of the distorted ether which are remote from the 

 oscillator AB, Fig. 184, do not follow the changes promptly. 

 This gives rise to electrical waves the nature of which at a dis- 

 tance from the oscillator is explained in a subsequent article. 



143. Examples of electric oscillators. The type of electric 

 oscillator which is described in Art. 142 was devised by Hertz 

 and used by him in his celebrated experimental researches on 

 electric waves in 1887.* An electric oscillator essentially similar 

 to the Hertz oscillator is employed as the sending device in 

 electric-wave telegraphy, wireless telegraphy so-called, as de- 

 scribed in Appendix D. 



Almost every electric spark discharge is oscillatory in character 

 as may be shown by photographing the spark upon a rapidly 

 moving photographic plate. Thus, a sharp flash of lightning 

 when photographed by means of a rapidly swinging camera 

 generally shows several parallel flashes very close together on 

 the photographic plate. The number of oscillations per second 

 of an electric discharge is, however, generally so great that the 

 sound of the spark cannot be distinguished from a sharp snap or 

 click. According to the principles enunciated in Appendix E, 

 however, it is evident that the number of oscillations per second 

 can be reduced to any desired value by increasing the inductance 

 of the circuit through which the discharge takes place and by 

 increasing the capacity of the condenser in which the charge is 

 stored. Thus, Fig. 185 shows a battery of Leyden jars JJ 

 arranged to discharge across an air gap g -and through a coil of 

 wire Z. , The sound produced by the spark in this case is a high 

 pitch musical tone of very short duration like the ringing sound 



* These researches are described in Hertz's book on Electric Waves, English 

 translation published by The Macmillan Company. 



