CHAPTER III. 



ELEMENTARY THEORY OF ELECTROMAGNETISM. 

 INDUCED ELECTROMOTIVE FORCE AND INDUCTANCE. 



23. Lenz's principle. The wires on a dynamo armature are 

 said to "cut" the magnetic lines of force as they move sidewise 

 across the magnetic field in the gap space between the field poles 

 and the armature core, and this "cutting" of the lines of force 

 induces an electromotive force in the armature wires. 



(a) In the motor, an electric current (from a battery, say) is 

 forced through wires which are allowed to move sidewise across 

 a magnetic field in the direction of the side push of the field on 

 the wires. This motion of the wires across the field induces in 

 them an electromotive force which opposes the flow of current, 

 and the work done by the battery in overcoming this opposition all 

 reappears as the mechanical work done by the side push as the 

 wires move sidewise. 



(b) In the generator, wires are pushed (by an engine, say) 

 across a magnetic field. This motion induces an electromotive 

 force in the wires, which in turn produces a current in the wires 

 and in the outside circuit to which the generator is connected. 

 This current causes the magnetic field to push sidewise on the moving 

 wires in a direction opposite to their motion, and the work done 

 by the engine in overcoming this opposition reappears as the elec- 

 trical work done by the electromotive force in maintaining the 

 induced current. 



The two statements (a) and (5) express a principle which was 

 first definitely pointed out by Lenz in 1834, and it is called Lenz's 

 principle. 



24. Law of induced electromotive force. Let be, Fig. 31, 

 be a wire stretched across a magnetic field as shown, the intensity 

 of the field being H gausses. The side force F (in dynes) 



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