454 BELL SYSTEM TECHNICAL JOURNAL 



3. Force on Current {Moving Coil Receivers, Light Valves, Motors) 

 A straight wire of length / carrying a current i in a perpendicular 



magnetic field of flux density B is pushed at right angles to the field 



and the length with a force 



F = Bil = txHil, 



where m is the permeability and H is the magnetizing force in the 

 nearby material from which the flux is derived. Again, the prime 

 requirement for a useful material is high flux capacity, and high 

 permeability, so that the magnetizing force need not be large. The 

 magnetizing force has been supplied generall^^in the past by means 

 of direct current in windings built into th^' apparatus. With the 

 normally available voltages and currents, sufficient magnetizing forces 

 could be obtained only with coils having a large number of turns, and 

 low resistance. This generally involved such large structures that 

 cost considerations compelled the use,6^f'iron cores with perhaps pole- 

 pieces made of permendur. Lately rnany structures are being designed 

 to replace costly electromagnets with' permanent magnets made from 

 Mishima type steel. 



Moving coil receivers and loud speakers " are the most important 

 representatives of this type of apparatus. Others are the string 

 oscillograph,'-* the light valve,-^ phonograph record recorder, ^"^ and 

 various types of power machinery. ^^ Several of these are now con- 

 structed with permendur pole-pieces, and cast Mishima steel magnets, 

 or remalloy magnets where hot rolling will assist in producing small, 

 accurately sized parts. 



4. Induced Electromotive Force ^ 



The electromotive force between the terminals of a coil of N turns 

 linking flux ip is 



^T d(p 



-^ = ^i- 



The arrangement of coils and interlinking flux difl'ers considerably in 



the various types of apparatus employing this effect. 



The flux variation is provided by means of mechanical motion of 



the coil in instruments such as the electromagnetic microphone. It is 



varied by means of fluctuations in magnetizing current in inductance 



" E. C. Wente & A. L. Thuras, B. S. T. J. 10, 565 (1931). 



28 A. M. Curtis, B. S. T. J. 12, 76 (1933). 



" G. E. Perreault, Bell Labs. Record 10, 412 (1932). 



30 H. A. Frederick, B. S. T. J. 8, 159 (1929). 



" R. D. deKay, Bell Labs. Record 16, 236 (1938). 



