216 



BELL SYSTEM TECHNICAL JOURNAL 



magnetic materials which ha\e a high pernieatiiHt\' ami a low coercive 

 force, but not necessarily capable of working at high densities. A 

 relatively high permeability reduces the energy- required to saturate 

 the core although due to the reluctance of the air-gaps there is obvi- 

 ously a limit beyond which no practical gain results due to increased 



P 



f^ 



Fig. 8 



permeabilities. The most importani single reeiuiremeiit of a magnetic 

 material for relays controlling light loads, is a low coercive force. A 

 low coercive force reflects the ability of the magnetic parts to return 

 to practically the same state of magnetization after repcatetl applica- 

 tions of magnetomotive forces. The effect of residual magnetism, if 

 large, may cause sticking or holding forces of the same order of mag- 

 nitude as the load requirements. Vacuum annealed silicon steels 

 of comparatively high silicon content and certain nickel steel alloys 

 which have low coercive forces are of great value for electromagnets 

 which must control cfticientK- lighl loads of the order of one to fifty 

 grams. 



\\lM)IN(; FORMUI..\K 



Before discussing the economics of the winding dimensions it is 

 necessary to develop and carefully consider the winding formulae 

 and the factors which determine the temperature characteristics. 



Fig. 8 shows the one-half cylindrical section of a spool. Since a 

 given wire ocrujiies a similar space in both A and B we need only to 

 consider winding space A. If rf in Fig. No. 8 represents the diameter 

 of the wire o\-er the insulation, it is evident that each wire may occii|n- 

 one of two positions with respect to adjacent wires. In the uniform 

 layup each wire occupies an area d", and with ilie c()m[)lcte inter- 



