142 ELEMENTS OF ELECTRICITY AND MAGNETISM. 



coil of wire having the same resistance as the lamp is connected 

 to the supply mains so as to take the same amount of current as 

 the lamp, and a much more intense spark is produced when this 

 circuit is broken ; an iron core consisting of a bundle of iron wires 

 is then placed in the coil and a spark several inches in length may 

 be produced by suddenly breaking the circuit. 



The kinetic energy of the electric current resides in the mag- 

 netic field which is produced by the current. Thus, a current in 

 the circuit a, Fig. 95, produces a very weak magnetic field except 

 in the small region between the wires, and the kinetic energy of 

 the current is small. The same current in the circuit b produces 

 an intense magnetic field inside of the coil and the kinetic energy 

 of the current is correspondingly great. The kinetic energy of 

 the current in the coil of wire shown in Fig. 95^ is much greater 

 than the kinetic energy of the same current in the circuit shown 

 in Fig. 95#, but the presence of the iron core in Fig. 95$ com- 

 plicates matters greatly, and nearly the whole of this chapter 

 relates to the kinetic energy of currents in the absence of iron. 



Practical applications of the spark at break. In the device 

 which is ordinarily used for lighting gas jets by electricity, an 

 electric circuit is made and broken in the stream of gas which is 

 to be lighted, and*the gas is ignited by the spark at break. In 

 order to produce an intense spark, the circuit includes a coil of 

 wire wound on an iron wire core, a so-called " spark coil." This 

 same device is used for igniting the mixture of gas and air in a 

 gas engine. 



76. Definition of inductance. The kinetic energy which is 

 associated with a current in a given circuit is proportional to the 

 square of the current ; that is, we may write 



W= \LP , (48) 



in which W is the kinetic energy of a current / in a given cir- 

 cuit, and (JZ-) is the proportionality factor. The quantity L is 

 called the inductance* of the circuit. 



* Sometimes called the coefficient of self-induction of the circuit. 



