ELEMENTARY THEORY OF ELECTROMAGNETISM. 67 



is shown in Fig. 51. The condenser C is charged from a high 

 voltage supply, ab, until the spark gap breaks down. Then the 

 charge on the condenser surges back and forth through the cir- 

 cuit CP. The coil P is the primary of a transformer of which 

 the secondary is the coil 5; the back and forth surging of current 

 through the primary circuit CP induces an alternating electro- 

 motive force in the secondary coil 5; and this induced electro- 

 motive force causes current to surge up and down in the antena. 

 The energy is then largely radiated from the antena in the form 

 of electric waves. Figure 52 shows a similar arrangement in 



antena 



1 



Po 



o 

 o 



Fig. 51. 



spark gap 



Fig. 52. 



which there is no appreciable radiation from the secondary 

 circuit SC f . The two circuits CP and SC' are said to be 

 coupled through the transformer PS. 



A mechanical arrangement which is completely analogous to 

 Fig. 52 is shown in Fig. 53 in which the lever, //', and the weight 

 W correspond to the transformer in Fig. 52; and the springs 

 C and C' correspond to the condensers in Fig. 52. 



The details of behavior of the coupled circuits in Fig. 52 can 

 be strikingly demonstrated with the help of the arrangement in 

 Fig. 53 as follows. The primary system PC has a definite 

 frequency of oscillation when the car 5 is held fast, and the 

 secondary system SC' has a definite frequency of oscillation 



