ADVANCED ELECTRICITY AND MAGNETISM. 



Equation (4) is plotted as a curve in Fig. 



Note. Something should be said as to how the propelling 

 electromotive force may be made to cease as assumed in the 

 above discussion of equations (2) and (4). It is not by breaking 

 the circuit, because to do this would increase the resistance R 

 indefinitely. In order that the current in a circuit may die away 

 according to equation (4) the battery must first be short-circuited 

 by a metal connector; and after being short-circuited in this 

 way the battery itself may be disconnected. 



38. The ratio of the inductances of two entirely similar coils 

 is equal to the ratio of their corresponding dimensions. To 



illustrate the meaning of this proposition consider two exactly 



Fig. 44. 



similar coils as shown in Figs. 44 and 45. The coil in Fig. 45 

 is twice as long and twice as deep, and wound with wire twice as 

 large in diameter as the coil in Fig. 44, and the inductance of the 

 coil in Fig. 45 is twice as great as the inductance of the coil in 

 Fig. 44. 



Of course the two coils in Figs. 44 and 45 have the same 

 number of turns of wire. 



The above proposition may be proven on the basis of equation 

 (i) in Art. 1 8, by showing that the field intensity at any point in 

 Fig. 45 is half as great as at the corresponding point in Fig. 44 



