450 



BELL SYSTEM TECHNICAL JOURNAL 



Reference to Figs. 1 and 2 points to 2 V-permendur as outstanding 

 in the flux density range above 15,000 gausses. This material is 

 excluded for many applications because of its high cost. High tem- 

 perature hydrogen annealing improves the high flux density behavior 

 of most magnetic materials, as noted above in connection with Fig. 2. 

 Using ordinary methods of annealing, the next best material for high 

 flux operation is the standard magnetic iron, while 45 permalloy is 

 preferable at flux densities below 12,000. For the low magnetizing 

 forces available in sensitive relays, 4-79 Mo-permalloy gives the largest 

 tractive forces. 



There are frequently other requirements in addition to tractive 

 force in relay construction. The operation and release characteristics 



Fig. 4 — U-type relay, showing the magnetic circuit 



are determined by the resistivity and coercive force of the material. 

 Sensitive, quick release relays require materials like permalloy, while 

 slow release types may utilize the larger coercive force and residual 

 of magnetic iron or cold rolled steel.^^- 23 



Relays for a-c. applications may have objectionable eddy current 

 losses in their cores. Such losses can be reduced by use of high 

 resistivity material. Thus, 45 permalloy has one-fourth the loss of 

 iron for the same core thickness and flux density. 



2. Polarized Tractive Force {Receivers, Ringers, Relays) 

 If permanent polarizing flux density Bp exists between two neighbor- 

 ing surfaces, and a small additional flux $ is applied by means of a 



22 H. N. Wagar, Bell Labs. Record 16, 300 (1938). 



23 F. A. Zupa, Bell Labs. Record 16, 310 (1938). 



