MAGNETIC MATERIALS IN TELEPHONE SYSTEM 455 



coils and transformers. In the moving coil microphone, the cylindrical 



coil moves axially in a slot between an inner magnetized cylinder and 



an outer magnetic cylinder which receives the radial flux threading the 



coil. For such a cylindrical coil in a uniform radial field, the e.m.f. is 



dx 

 — e = IB jr , where / is the total length of wire composing the coil, 



and X is its displacement perpendicular to the radial field. For a 



stationary coil linking a varying flux as in a transformer, — e — AN -j- 



= A Nil -^- , where A and /x are the area and permeability of the core 

 at 



within which the magnetizing force is H. It is evident from these 



equations that high flux density or permeability are desirable, in order 



to yield the largest e.m.f. with least material. 



4a. Microphones, Magnetic Tape Recorders, Magnetos 



An application involving a moving coil is the public address micro- 

 phone,^^ where the flux is established by means of a cobalt steel magnet, 

 and is concentrated upon the moving coil by means of permendur 

 pole-pieces. 



An inverse application in which the coil is stationary and the magnet 

 moves is the magnetic tape recorder. ^^ In this, a steel tape which has 

 been magnetized by speech currents is drawn between permalloy pole- 

 pieces in pick-up coils. High coercive force, high signal-to-noise ratio, 

 mechanical soundness, durability, and cheapness of the tape material 

 are desirable. 



The telephone magneto employs the e.m.f. generated by rotating a 

 coil in a magnetic field. It has been constructed with iron armature 

 and pole-pieces, and chrome steel field magnets. Recent designs 

 using modern magnetic materials have indicated the possibility of 

 large economies in volume.*^ The magnetic properties of available 

 materials have now reduced the volume required by magnetic parts to 

 a point where the major problem in magneto design is to compress 

 the gears and shafts into correspondingly small space and yet main- 

 tain sufficient mechanical strength, durability, and convenience of 

 operation. 



4b. Inductance Coils 



A very important application of induced e.m.f. focuses attention on 

 the inductance of a coil of N turns surrounding a (closed) core of area A , 



32 R. N. Marshall & F. F. Romanow, B. S. T. J. 15, 405 (1936). 



33 C. N. Hickman, B. S. T. J. 16, 165 (1937).' 



34 Ericsson Bulletin No. 12, 46 (1938). 



