MAGNETIC MATERIALS IN TELEPHONE SYSTEM 451 



magnetomotive force M, the additional tractive force will be approxi- 

 mately 



Rail + njHr) 



where i?„ in the latter part of the above equation is the reluctance of 

 the air-gap, n is ratio of the reluctance of the ferromagnetic circuit 

 with iron removed to the reluctance of the air-gap, and iir is the 

 reversible permeability, i.e. the permeability measured with very small 

 a-c. magnetizing forces in the presence of the polarizing flux density. 

 It thus appears that materials for such applications should have 

 high saturation values. The apparatus should be designed to obtain 

 a low value of w, and to operate at a flux density to make the above 

 force a maximum. 



Figure 5 gives values of ^r as a function of polarizing or superposed 

 flux density Bp. It should be remarked that the reversible permea- 

 bility is practically single valued ^"^ when plotted against polarizing 

 flux, in contrast with the "butterfly" loop obtained by plotting against 

 polarizing field strength. The superiority of permendur in Fig. 5 is 

 obvious. Values of the force factor for n = 100 and n = 1000 have 

 been computed for these materials for an arbitrary value of air-gap 

 reluctance and magnetomotive force. It is evident that the full 

 advantage of permendur is not realized unless the apparatus design 

 is such as to attain a low value of the air reluctance ratio n. 



Permanent magnet materials are frequently employed to supply 

 polarizing flux. When they form a part of the circuit for the alter- 

 nating flux, their reversible permeability becomes important. Refer- 

 ence to Table II shows that 5 per cent W steel has the highest permea- 

 bility of the common permanent magnet materials. However, its 

 energy product {B'H)max. is so low that other materials are preferred 

 for applications where space is limited, despite their low reversible 

 permeabilities. 



The earliest application of polaiized structures in the telephone 

 plant was to the receiver. The receiver of the present day is con- 

 structed with remalloy permanent magnets, 45 permalloy pole-pieces, 

 and a permendur diaphragm.-^ The magnetic circuit is shown in 

 Fig. 6. 



The second application of polarized structures is to the telephone 

 bell or ringer. Here a permanent magnet is used, to supply polarizing 

 flux to the two cores for the coils through the shoe at one end and the 



^* R. Cans, Phys. Zeit. 12, 1053 (1911). 

 25 W. C. Jones, B. S. T. J. 17, 338 (1938). 



