ELECTROMAGNETS. MAGNETISM OF IRON. 3/5 



is given. The flux from a north pole piece to a south pole piece 

 consists of two parts : (i) The useful flux <l>, which passes from 

 a north pole face across an air gap, through the armature core, 

 and across a second air gap into a south pole face, and (2) The 

 leakage flux <, which passes out from all portions of the surface 

 of a north pole piece, other than the pole face, into the similar 

 portions of the surface of a south pole piece. 



To calculate <f> proceed as follows : Knowing <E> find the flux 

 density in the armature core, then take from tables (or curves) 

 the corresponding value of fi for the ar- 

 mature core, and calculate the magnetic 

 reluctance of the armature. Next cal- 

 culate the magnetic reluctance of each 

 gap space between pole faces and arma- 

 ture core. Add these three reluctances 

 together and multiply by <I> to get the 

 magnetomotive force from pole piece to 

 pole piece. Let this be represented by &. 



The leakage flux <f> is the sum of the fluxes flowing between 

 the surfaces I I, between the surfaces 2 2 (at both ends of the 

 machine), and between the surfaces 3 3, as shown in Fig. 15. 

 Calculate the magnetic reluctance of each one of these elementary 

 leakage paths with the help of equations (12) and (13). Divide 

 cf by the reluctance of each path to find the leakage flux through 

 each path. Finally, add these various leakage fluxes together to 

 get</>. 



(b) When magnetic leakage is due to an air gap and an oppos- 

 ing magnetomotive force combined. Magnetic leakage around a 

 dynamo armature when the armature current is not zero. Let 

 <E> be the flux through the armature, <j> the leakage flux, ef the 

 magnetomotive force between the pole pieces, and &' the oppos- 

 ing magnetomotive force due to the current in the armature. 

 The flux 4> is supposed to be given. 



To calculate <f> proceed as follows : Find the magnetic re- 

 luctance of the armature core and of the gap spaces exactly as 



