62 



ADVANCED ELECTRICITY AND MAGNETISM. 



This gives the inductance of one mile of the line in abhenrys. 

 To reduce to henrys divide by io 9 , and then reduce the Naperian 

 logarithm to a common logarithm and we get equation (i). 



40. Mechanical analog of the transformer. The use of the 

 lever for the multiplication of mechanical force is familiar to 

 everyone, and the alternating-current transformer is mathematically 

 analogous to the lever; not however to a lever with an immovable 

 fulcrum, but to a lever of which the fulcrum is a very massive body 

 entirely free to move as shown in Fig. 49 



^ /' 



heavy body 

 of mass M 



o 



Fig. 49. 



A steady force would even- 

 tually produce an indefinitely 

 large velocity of the body M 

 (wheels supposed to be friction- 

 less). Therefore the lever 

 shown in Fig. 49 can be used 

 only for multiplying a rapidly 

 alternating force (applied to 

 end A). 



Imagine a very rapidly alter- 

 nating force applied to end A 

 of the lever in Fig. 49. The 



A steady electromotive force 

 would eventually produce an 

 indefinitely large current 

 through the primary coil of a 

 transformer (resistance of coil 

 assumed to be very small). 

 Therefore the transformer can 

 be used only for multiplying a 

 rapidly alternating electromo- 

 tive force (applied to the 

 primary coil). 



An alternating electromotive 

 force is applied to the primary 

 coil A of a step-up trans- 



