3/2 ELEMENTS OF ELECTRICAL ENGINEERING. 



remote from the seat of the magnetomotive force, tends to spread 

 out and pass through the surrounding air as well as through the 

 clearly defined path which is provided for it. This effect is espe- 

 cially prominent when, as in case of the dynamo, the portion B 

 of the magnetic circuit contains an air gap, and also when there 

 is an opposing magnetomotive force in the portion B of the mag- 

 netic circuit. Thus in the dynamo the demagnetizing action of 

 the armature current constitutes an opposing magnetomotive 

 force. 



Let 4> be the magnetic flux which enters the armature from a 

 north pole of a field magnet and leaves the armature at a south 

 pole of the field magnet. This is called the useful flux. A cer- 

 tain amount of flux </> passes through the air from each north 

 pole of the field magnet to the adjacent south pole, or poles, 

 without passing through the armature. This is called the leak- 

 age flux. The total flux, <I> + </>, passes through the field mag- 

 net cores.* The ratio (<& + $)/<> is called the coefficient of 

 magnetic leakage of the dynamo. It is always larger than unity 

 and it ranges in value from 1.125 to as much as 2.0. That is, 

 the leakage flux (j> is in some cases as small as 0.125 of the use- 

 ful flux, and in some cases it is equal to useful flux. The coef- 

 ficient of magnetic leakage of a dynamo may be determined 

 accurately by experimental tests on the machine, or, if necessary, 

 it may be approximately calculated from the known dimensions 

 of the various parts of the machine. 



Experimental determination of magnetic leakage coefficient. 

 The armature of the dynamo to be tested is kept stationary dur- 

 ing the test. The terminals of the field winding are connected 

 to supply mains through a rheostat and a reversing switch. The 

 object of the rheostat is to adjust the field excitation to the de- 

 sired value, and the reversing switch is used for quickly reversing 

 the field excitation. A number of turns of fine wire are wound 



*Ir> the field magnet structures shown in Figs. 32, 33, 34, 37 and 39, the whole 

 of 4> -f- <?> passes through each magnet core C. In the structures shown in Figs. 35, 

 36 and 38, half of <J> -(- $ passes through each magnet core C. 



