254 BELL SYSTEM TECHNICAL JOURNAL 



very low frequencies is improved by the use of permalloy. Sheet 

 permalloy has been followed by compressed powdered permalloy^ and 

 this by perminvar,^ the newest member of the magnetic alloy family. 

 Compressed powdered permalloy has replaced the powdered iron as 

 it has all of the desirable properties of the latter and to an even greater 

 degree. By virtue of higher permeability combined with lower hys- 

 teresis loss, it has made possible the design of smaller coils of superior 

 performance characteristics. As an illustration the two loading coils 

 of Fig. 18 are shown, the smaller of these being the electrical equivalent 



Fig. 18 — Relative size of powdered iron (left) and powdered permalloy (right) 



loading coils. 



of the larger in all respects and in some its superior. In general the 

 reduction in coil size made possible by the use of powdered permalloy 

 in place of powdered iron amounts to about 75 per cent giving very 

 substantial savings in manufacturing costs, handling problems and 

 installation space required. 



Perminvar is remarkable in an entirely unique respect. Its permea- 

 ability is not exceptionally high, being of the same order as that of 

 ordinary soft iron at moderately low magnetizing forces, but it is 

 exceptionally constant with respect to magnetizing forces. This is 

 shown in Fig. 19 from which it will be noted that there is substantially 

 no change in permeability up to a force of about 2 gausses whereas over 

 this same range, the permeability of soft iron undergoes a change of 

 more than 2,000 per cent. Up to somewhat smaller magnetizing forces, 

 perminvar has a vanishingly small hysteresis loss. Fig. 20 depicts this 

 loss for perminvar. It is to a material of constant permeability and 

 low hysteresis loss that the transformer designer turns when he has a 

 difficult requirement as to low modulation to meet. Unfortunately, 

 while perminvar has these properties over a limited range of magnetiza- 



' "Compressed Powdered Permalloy, its Manufacture and Magnetic Properties," 

 W. J. Shackelton & I. G. Barber, Trans. A. I. E. E., Vol. 17, 1928. 



* "Magnetic Properties of Perminvar," G. W. Elmen, Jotir. of Franklin Institute, 

 Vol. 206, 1928. 



