ELECTRIC OSCILLATIONS AND ELECTRIC WAVES. 255 



which is produced by striking an anvil with a hammer. The 

 pitch of the " ringing spark " may be raised by decreasing the 

 number of turns of wire in the coil Z, or by decreasing the 



Fig. 185- 



number of Leyden jars in the battery JJ. The oscillatory char- 

 acter of the spark across the gap g may be shown by viewing 

 it in a rotating mirror, and in this way ten or more images of the 

 spark may be seen side by side at each discharge of the battery 

 of Leyden jars. 



The oscillatory discharge of a condenser through a coil of wire 

 is utilized in a type of induction coil which is due to Nikola 

 Tesla. A helix PP, Fig. 186, of ten or fifteen turns of coarse 

 wire is connected to the 



o 



terminals CD of a 

 charged condenser AB, 

 or battery of Leyden 

 jars, with a spark gap in 

 the circuit at g. The 

 condenser is connected 



to the secondary of a high voltage transformer, each high volt- 

 age impulse of the transformer charges the condenser until the 

 air gap at g breaks down, and then the condenser charge 

 surges back and forth through the helix PP until the energy of 

 the charge is dissipated. A jet of air * issues from a nozzle J 

 and blows away the air which has been heated and ionized by 



* By using zinc terminals for the spark g and by placing two or three very short 

 spark gaps in series the air jet becomes unnecessary. 



A 



TO TRANSFORMER I 



Fig. 186. 



