ELEMENTARY THEORY OF ELECTROMAGNETISM. 47 



Z" 



sents e f , and E" is -=7 as long as ', as shown in Fig. 38. 



Zj 



Figure 38 applies to a transformer in which Z' = 2Z". 



31. Magnetic flux through the opening of a circuit due to the 

 current in the circuit. Definition of inductance. Consider a 

 coil consisting of a single turn of wire as shown in Fig. 39. The 



wire 



front view 



Fig. 39. 



lines of force of the magnetic field due to a current in the coil are 

 shown in the side view. The amount of magnetic flux S> through 

 the opening of the coil is proportional to the current / in the 

 coil. That is, we may write: 



$ = LI (i) 



where L is a proportionality factor which depends on the size 

 and shape of the circuit. It is called the inductance of the circuit. 



Equation (i) is exactly true only when there is no iron in the 

 neighborhood of the coil. Therefore, the remainder of this 

 chapter applies strictly to electric coils or circuits without iron. 



It is best to use c.g.s. units (abamperes, abvolts, etc.) in any 

 equation involving magnetic field intensity H or magnetic flux 

 $, because the units of H and <i> which are generally used, 

 namely, the gauss and the maxwell (or line of force), are c.g.s. 

 units. The c.g.s. unit of inductance is called the abhenry, and 

 L in equation (i) is expressed in abhenrys when $ is expressed 

 in maxwells (lines of force) and I in abamperes. 



When the coil has more than one turn of wire the expression 



