456 BELL SYSTEM TECHNICAL JOURNAL 



magnetic path length /, and permeabiHty //. The inductance is in- 

 creased through the presence of the core by an amount 



L = 47riVV^//. 



As noted earlier, when eddy current shielding is negligible such an 

 inductance is accompanied at frequency/ by hysteresis, residual, and 

 eddy current resistances to give a total as follows: 



RIL = ixJ{aB,n + f + e/). 



At frequencies high enough to introduce eddy current shielding, 

 the effective inductance due to a core of laminar thickness / and 

 resistivity p is reduced below the ordinary inductance Lo by the ratio 



TIT — ^ ^'"^ ^ "I" ^^" ^ 

 ' "^ ~ "d cosh e + cos ' 



where 6 = iTrt^nof/p. Figure 7 shows this ratio and the ratio Re/wLo 

 as functions of 6. As a practical example, the permeability of 6 mil 

 (0.015 cm.) 4-79 Mo-permalloy is reduced to about 75 per cent of its 

 initial value (22,000) at 1 kilocycle, and to about 17 per cent at 

 10 kc. 



The telephone loading coil adds inductance to the telephone line, 

 but it must not add excessive resistance. Furthermore, its inductance 

 must be extremely stable with the lapse of time, and under severe 

 operating conditions, such as occasional current surges induced from 

 lightning discharges. Iron wire cores for loading coils were supplanted 

 over twenty years ago by compressed iron powder cores, and these in 

 turn gave way to permalloy powder cores. The latest improvement 

 is the introduction of 2-81 Mo-permalloy powder cores.'' The reduc- 

 tion in size of cores with these improvements is shown in Fig. 8. 



Another method of loading a line is by sheathing the conductor with 

 a continuous layer of magnetic material. This method was used 

 with notable success on submarine telegraph cables by wrapping 

 permalloy tape upon the conductor, and annealing before applying 

 insulation.^ The location of the loading material is shown in Fig. 9. 

 The continuous loading of long submarine telephone cables has been 

 shown to be feasible using thin 7.5-45 Mo-perminvar tape.^ 



Retardation and choke coils have a great variety of applications, 

 running from a tiny coil weighing 3^ ounces ^* to a 4600 lb. generator 

 ripple suppressor.^*^ The contrast is evident in Fig. 10. Retardation 



35 D. W. Grant, Bell Labs. Record 11, 173 (1933). 

 3« R. A. Shetzline, Bell. Labs. Record 17, 34 (1938). 



