460 BELL SYSTEM TECHNICAL JOURNAL 



4b apply. In addition, however, the coupUng factor between coils on 

 the same core becomes of especial importance. In the usual design 

 the flux linkage common to the primary and secondary is largely con- 

 tained in the magnetic material, while the leakage flux is controlled by 

 the reluctance of the air path. It is thus evident that a large value 

 of core permeability is required to obtain a high coupling factor. 

 Of course, advantages indicated by high permeability may be lost 

 through improper design. 



The earliest transformer employed in the telephone plant was the 

 induction coil. This originally consisted of two windings on a core 

 composed of a bundle of iron wires. ^^ Later, silicon iron sheet cores 

 were introduced. 



Input and output transformers have varied applications for which 

 special types of cores are required. Where superposed current is not 

 involved, space and weight can be economized by use of high perme- 

 ability materials such as chrome or molybdenum permalloy. ^^ Eddy 

 current shielding at higher frequencies will offset much of the perme- 

 ability advantage indicated for these materials unless they are lami- 

 nated sufficiently. However, thin laminations are costly to prepare 

 and stack, and difficult to insulate and handle without mechanical 

 injury and corresponding reduction of permeability. An intermediate 

 thickness of permalloy sheet is generally chosen, which secures a 

 considerable advantage in permeability over iron, without prohibitive 

 cost. 



Where a winding must carry direct current, conditions are similar 

 to those applying to choke coils, and materials with high reversible 

 permeability at high magnetizing forces are required. Frequently, 

 for large magnetizing forces, it is desirable to include air-gaps in the 

 magnetic circuit. Silicon iron is the ordinary material for such 

 application, as it is for power transformers. 



4d. Magnetic Shielding 



A further application of induction effects is in magnetic shielding 

 of apparatus. A magnetic shield consists of a high permeability shell 

 (4-79 Mo-permalloy, or 78.5 permalloy) which shunts flux around the 

 enclosed apparatus. For a-c. shielding, alternate layers of copper and 

 permalloy sheet are very effective in magnetic shunting and eddy 

 current screening of the enclosed space.""^ High initial permeability, 



^* Cf . p. 43 of Reference 26. 



39 A. G. Ganz & A. G. Laird, Elec. Engg. 54, 1367 (1935). 



« W. G. Gustafson, B. S. T. J. 17, 416 (1938). 



