I 54 ELEMENTS OF ELECTRICITY AND MAGNETISM. 



will jump across the air gap instead of going through the coil 

 AB. Thus, if the air gap is one centimeter in length it takes 

 20,000 volts to strike across it, and if a spark does strike 

 across this gap at the instant of the discharge of the Ley den jar, 

 one may be certain that the electromotive force between the ter- 

 minals of the coil was at least 20,000 volts at the instant of the 

 formation of the spark. 



The protective action of the choke coil in Fig. 99 depends 

 upon a rapid increase of current through the coil during an ex- 

 tremely short interval of time just before the gap g breaks down. 

 The dynamo G may be protected from this very brief flow of 

 current by connecting a condenser; between the point a and 

 earth, so that this very brief flow of current through the choke 

 coil need not flow through the dynamo, but may go to charge the 

 condenser. 



81. Inductance of a long solenoid. A solenoid is a long coil 

 of wire ; two or three layers of wire wound on a long wooden 

 rod, for example. When the depth of the winding of wire is 

 small in comparison with the radius of a solenoid and when the 

 length of the solenoid is great in comparison with the radius, the 

 inductance of the solenoid in abhenrys is given by the following 



equation 



(55*) 



in which L is the inductance of a solenoid in abhenrys, z is 

 the number of turns of wire on each centimeter of length of the 

 solenoid, r is the radius of the solenoid in centimeters, and / is 

 the length of the solenoid in centimeters. The inductance of 

 the solenoid in henrys is given by the equation 



in which r and / are expressed in centimeters as in equation 



(55*)- 



Derivation of equation (55). The intensity of the magnetic 



field inside of the solenoid is // ^.irzl, according to equation 



