MAGNETIC DKSKiX OF RELAYS 



37 



As illustrated in Fig- <>, i^ and // are therefore dotciniincd by the inter- 

 section witli the demagnetization curve of the line luuing a sIojk' gix-en 

 by the right-hand side of (9). The complete magnetic circuit has a 

 residual flux ^p = Ba resulting from the coercive mmf, //c^, which equals 

 the sum of the potential drop II({ — (S{<p) external to the coi'e and the 

 drop (Ifc-H)f\n the core. 



Propcrfics of M (uinclic Mulvrials 



'V\\v properties of the magnetic materials commonly used for electro- 

 magnets are shown in Table 1. This includes, in addition to the magnetic 

 pioperties, the working properties in manufacture, and the resistivity. 

 The latter determines the eddy current delay in otherwise similar struc- 

 tures, and is a controlling factor where fast release is desired. 



For design purposes, allowance must be made for variations in tlie 

 material, which are relati\'ely lai'ge in the case of magnetic properties. 

 Minimum and maximum values are therefore given in Table L As indi- 

 cated by the footnote in this table. He is subject not only to initial 

 variation, but to an increase with time: "aging." The relatively large 

 magnitude of this change for magnetic iron is a serious disadvantage in 



8 



7 



10 

 D 

 < 



O 5 



40 30 20 



-H IN OERSTEDS 



0.05 0.10 0,15 0.20 25 

 BH IN ERGS xlO^ 



Fig. 6 — Demagnetizntion and energy i)ro(luct curves (1}^ per cent chrome 

 steel). 



