36 ELEMENTS OF ELECTRICAL ENGINEERING. 



stretched across a field of intensity &C. This current causes the 

 field to push the wire sidewise with the force 



F=IIM dynes 



This force moves the wire sidewise at a velocity of v centimeters 

 per second and develops mechanical power. The motion of the 

 wire induces in it the electromotive force 



E = IWv abvolts 



which opposes the flow of current in the wire. Work is done by 

 the external source of electric current at the rate of El ergs per 

 second in forcing the current against the electromotive force E, 

 and the work so spent goes to maintain the motion of the wire. 

 There are two distinct types of dynamo electric machines, 

 namely (a) alternating current machines, and (^) direct current 

 machines. The alternating current generator delivers what is 

 called an alternating current, that is, a current which is subject 

 to rapid periodic reversals of direction. The direct current gen- 

 erator, on the other hand, delivers a current which is not reversed 

 in direction and which is more or less steady in value. All direct 

 current machines, except the so-called homopolar dynamos, are 

 essentially alternating current machines with the addition of a 

 commutator instead of collecting rings. 



27. The alternating current dynamo is the simplest form of 

 dynamo electric machine. It consists essentially of an arrange- 

 ment in which a loop of wire is moved in front of the projecting 

 north and south poles of a magnet in such a way that the mag- 

 netic flux due to the magnet poles passes through the loop first 

 in one direction and then in the other direction repeatedly. This 

 repeated reversal of the magnetic flux through the loop of wire 

 induces an electromotive force in the wire in one direction while 

 the flux is increasing (from negative to positive), and in the other 

 direction while the flux is decreasing (from positive to negative). 

 This alternating electromotive force produces an alternating cur- 



