ELECTRIC OSCILLATIONS AND ELECTRIC WAVES. 263 



When one end of a stretched rubber tube is held in the hand 

 and moved up and down slowly, the entire tube has time to 

 accommodate itself to the changing position of the hand. If, 

 however, the hand is moved up and down rapidly, the portions 

 of the tube remote from the hand do not follow the changing 

 position of the hand promptly, and the result is that waves are 

 produced which pass out from the moving hand. The oscillatory 

 changes above described in connection with Fig. 204 take place 

 so rapidly that the portions of the distorted ether which are 

 remote from the oscillator do not follow the changes promptly. 

 This gives rise to electrical waves which pass out from the 

 oscillator. In the immediate neighborhood of the oscillator the 

 action is rather complicated, but at a distance from the oscillator 

 the wave motion becomes very simple.* 



PROBLEMS. 



1. A Leyden jar has walls 2 millimeters thick, the area of 

 each tinfoil coating is 500 square centimeters, and the induc- 

 tivity of the glass is 5. What is the capacity of the jar? Ans. 

 o.oon microfarad. 



2. The Leyden jar of problem I is discharged through a circuit 

 which contains a coil of wire which consists of a winding of 1 ,000 

 turns of wire on a cylindrical wooden rod 6 centimeters in diam- 

 eter and 1 20 centimeters long. What is the inductance of the 

 circuit and what is the frequency of the electrical oscillations 

 which follow the discharge? Ans. 0.00296 henry; 882 ,000 cycles 

 per second. 



Note. The influence of resistance on the frequency is to be ignored. 



* Hertz's researches on electric waves, experimental and theoretical, have been 

 published in book form (see Electric Waves by Heinrich Hertz, translated by D. E. 

 Jones, Macmillan and Company, 1893). 



A very good discussion of Hertz's experimental researches is given by J. A. 

 Fleming on pages 306-326 of his Principles of Electric Wave Telegraphy, Longmans, 

 Green and Company, 1908. 



A good discussion of the mathematical theory of the Hertz oscillator is given 

 by Fleming on pages 326-352 of his Principles of Electric Wave Telegraphy. This 

 theoretical discussion of Fleming's follows the original paper by Hertz which was 

 published in 1889 (see pages 137-159 of Hertz's Electric Waves). 



