ELECTRIC MOMENTUM. INDUCTANCE. 



153 



out (which is not shown in the figure) constitute what is called a 

 lightning arrester.* 



Example. A coil of heavy copper wire wound in a single 

 layer on a wooden cylinder AB, as shown in Fig. I oo. is provided 

 with two metal rods rr which are separated by a small air gap. 

 One terminal of the coil is connected to the outside coating of a 

 Leyden jar, and a spark is allowed to jump from the jar to the 



Fig. 100. 



other terminal of the coil as shown in the figure. At the instant 

 of formation of the spark the total electromotive force between 

 the coatings of the Leyden jar begins to act on the circuit. If 

 the coil consists of 100 turns of wire wound on a wooden cylin- 

 der 4 centimeters in diameter and 30 centimeters long, its ap- 

 proximate inductance is 0.00005 henry, so that, if the electromo- 

 tive force between the coatings of the Leyden jar is 40,000 volts, 

 the current begins to increase in the coil at the rate/of 800,000,000 

 amperes per second, according to equation (49).! The existence 

 of a large electromotive force across the terminals of the coil 

 may be shown by the fact that the discharge of the Leyden jar 



* For further information concerning lightning arresters see Franklin and Esty, 

 Elements of Electrical Engineering, Vol. I, pages 210-219. 



