ELECTROMAGNETS. MAGNETISM OF IRON. 367 



subjected to a uniform magnetizing force 3f along its entire length, 

 the value of the flux density cB in the rod may be found from the <$ 

 and c*f curve, and the total flux through the rod may be found 

 by multiplying this flux density by the sectional area of the rod. 

 Or, the uniform value of &C required to give a specified total flux 

 through the rod may be found from the cB and &C curve after 

 the value of $ has been found by dividing the specified flux by 

 the sectional area of the rod. 



When, however, the iron rod varies in size from point to point, 

 or when different parts of the rod are of different kinds of iron, or 

 when there is an air gap in the magnetic circuit, then some kind 

 of an averaging process must be used to calculate the magneto- 

 motive force required to produce a specified flux through the rod, 

 or conversely to calculate the flux produced by a specified mag- 

 netomotive force. 



This calculation is carried out as follows : 



(a) To find the magnetomotive force required to produce a speci- 

 fied flux. Divide the prescribed flux by the sectional area in 

 square centimeters of each part of the circuit, wrought iron, cast 

 iron, or air, as the case may be. This gives the flux density cB 

 in each part of the magnetic circuit.* Knowing cB for each part 

 of the circuit, take from the <8 and 3f tables or curves for the 

 different kinds of iron, the value of 3f required to produce the 

 known value of cB in each part of the circuit, and multiply the 

 value of 3{ for each part of the circuit by the length in centi- 

 meters of that part. This gives the magnetomotive force re- 

 quired for each part, and the sum of these magnetomotive forces 

 is the total magnetomotive force required. This total magneto- 

 motive force may then be reduced to ampere-turns by multiply- 

 ing it by IO/47T. 



In this calculation it is to be remembered that flux density in air 



* It is here assumed that the whole of the magnetic flux passes through each por- 

 tion of the iron circuit. This is not always the case, for a portion of the magnetic 

 flux may stray through the air. This portion is called the leakage flux. Magnetic 

 leakage is discussed in a subsequent article. 



