36 



THE BELL SYSTEM TECHNICAL JOURNAL, JANUARY 195- 



H = 0. To account for this there must be added to the magnetic postu- 

 lates cited above the assumption that there may be a movement of 

 electric charge within the material which suppUes an effective mmf per 

 unit length measured by the coercive force He , represented in Fig. 4 by 

 the distance from the origin to the point 4. 



When the coil circuit is opened in relay release, the coercive force of 

 the core material results in a residual flux passing through the air gaps 

 in series with the iron path. A potential drop J must exist across these 

 gaps. In the absence of applied magnetomotive force, this must be bal- 

 anced by an equal and opposite drop through the core. There is thus an 

 imposed negative potential gradient —H per unit length of the core. 

 The remanence Br is therefore governed by the B—H relation in the 

 second quadrant, the curve 3-4 of Fig. 4. This is the demagnetization 

 relation, as illustrated separately in Fig. 6. The intercepts of this curve 

 on the B and H axes are He and Br . 



If (Re is the reluctance of the magnetic circuit external to the core, the 

 external drop ^e equals 6{e(P, where <p is the flux. Taking B and H as 

 uniform in the core, <p = Ba and J equals H(, where a and / are the cross- 

 sectional area and length of the core. The values of B and H in the core 

 must therefore conform to the equation : 



B 

 H 



a(R, 



(9) 



6 



XIO^ 



5432 100 1 2 34 5 



PERMEABILITY, >X H IN OFRSTEDS 



Fig. 5 — M-B and B-H curves for magnetic iron. 



