2 $6 ELEMENTS OF ELECTRICITY AND MAGNETISM. 



the spark. Then charge can again accumulate on the condenser 

 until a new discharge takes place. The successive discharges 

 may be as frequent as several thousand per second (a number of 

 successive discharges taking place during each high voltage im- 

 pulse of the charging transformer), and the oscillations of each 

 discharge may be at the rate of a million or more per second. 



A second helix 55 * of several hundred turns of wire sur- 

 rounds the helix PP (not so shown in the figure), that is, the coils 

 PP and 55 constitute the primary and secondary coils of an 

 induction coil. The rapidly oscillating current in PP due to 

 the discharge of the condenser induces very large electromotive 

 forces in 55 and produces long sparks between the terminals 

 of 55. 



A very striking property of the discharge from 55, which is 

 due to its high frequency, is that it traverses only the surface 

 layers of a conductor and it may therefore be passed through 

 (over) the human body with impunity. 



Leyden jars as oscillators and resonators. Similar circuits may 

 be connected to two Leyden jars so that the oscillations which 

 occur when one Leyden jar discharges through its circuit are in 

 unison with the proper oscillations of the closed circuit of the other 

 jar, so that the inducing action on the circuit of the second jar is 

 cumulative. An instructive experiment is the following : A Ley- 

 den jar is connected to a vertical rectangular circuit of wire ww as 

 shown in Fig. 187, and an electric machine repeatedly charges the 

 jar until it discharges across the air gap g and through the circuit 

 ww. This discharge is oscillatory in character and it has a 

 definite frequency. A second jar similar to the first is short-cir- 

 cuited by a vertical rectangular wire frame ww as shown in Fig, 

 1 88, and placed along side of the arrangement shown in Fig. 

 187. By adjusting the size of the circuit in Fig. 188, the free 

 period of oscillation of this circuit may be made to coincide with 

 the period of oscillation of the circuit in Fig. 187, and, when 

 this condition is reached, the induced oscillations in the circuit 

 become sufficiently intense to produce a spark across the air gap 



