INDUCED ELECTROMOTIVE FORCE. 119 



work is required to keep the span turning than would otherwise be 

 necessary, and the work received by the moving person is equal to 

 the additional work so expended in turning the span. 



M of rotation 



Fig. 81. 



Inasmuch as the idea of electric current strength is strictly 

 analogous to the mechanical idea of velocity, the question as to 

 what becomes of the work done in moving a wire against a force 

 ivhich depends on the current is strictly analogous to the question 

 as to what becomes of the work done in moving a body against a 

 force which depends on velocity. Therefore the above example of 

 a person moving radially on a swinging bridge span is analogous 

 to the following : A wire is connected to a battery so that an 

 electric current flows through it, and the wire is stretched across 

 a magnetic field as shown in Fig. 6 1 . Under these conditions the 

 magnetic field pushes sidewise on the wire, and this side force 

 depends on the current. If the wire be moved sidewise against 

 this force, work has to be done and this work helps to maintain the 

 current. If the wire is moved in the direction of the side force ', the 

 side force does work in helping to move the wire, more work is re- 

 quired to keep the current flowing than would otherwise be neces- 

 sary, and the work received by the moving wire is equal to the ad- 

 ditional work thus done in keeping the current flowing. 



In the example of the swinging bridge span, the force exerted 

 by the engine which drives the span must be supposed to be 

 greater or less according as the man is moving outwards or in- 

 wards (with or against the centrifugal force) if the velocity of 

 turning is to be kept constant. In the example of the moving 



