VOL.2, No. 12
PERMALWY
1157
PERMALLOY i
BELL TELEPHONE LABORATORIES, NEW YO=, N. Y.
The name "permalloy" is applied to certain nickel-iron alloys which, when properly heat-treated, possess remarkable magnetic properties. These properties are developed in alloys which contain more than 30 per cent of nickel and the atoms of which have the face-centered cubic arrangement characteristic of nickel rather than the body-centered crystal structure characteristic of iron. While alloys throughout the entire range above 30 per cent nickel exhibit these properties to some degree, the most remarkable magnetic effects are obtained with alloys containing nickel and iron in the ratio of about 80 per cent nickel to 20 per cent iron. In the manufacture of permalloy it is desirable to use the purest commercial nickel and iron. The analysis of one typical sample in per cent was as follows: Ni78.23;Fe 21.35;C0.04; Si 0.03;Ptrace; S 0.035; Mn 0.22;Co 0.37 and Cu 0.10. The heat treatment of permalloy is of the utmost importance. To develop its highest permeability i t must he cooled not only through the proper temperature ranges but also a t the proper rates. Each size and shape of finished product has its own best heat treatment. A magnetizable material placed in a magnetic field has established in i t a magnetic induction, the amount of which depends upon the intensity of the field to which it is exposed and upon a characteristic of the substance known as its permeability. This permeability is the factor by which the magnitude of the cause, namely the field, must be multiplied in order
1158
JOURNN.
oa C m m w EDUCATION
DRcEmSn. 1925
to obtain the effect, namely the induction. Permeability, however, is not a constant factor independent of the magnetic condition of a ferromagnetic substance. For example, doubling the magnetic field does not necessarily double the magnetic induction since the ease of magnetization, that is the permeability, may be markedly greater or less under this new condition. It is in its initial permeability and its ease of magnetization under very feeble magnetic fields that permalloy is most remarkable. So sensitive may it be made that i t will be saturated magnetically in the earth's magnetic field. The magnetic properties of permalloy are dected by strain; for example, in a particular case the maximum permeability may be reduced from its value in the unstrained condition to about one-tenth that value merely by variations in tension within the elastic limit. The electrical conductivity of the material is similarly sensitive to strain through a much smaller range of only a few per cent, but a t a rate which is large as compared with that of other metals. In electrical communication relatively intense magnetic effects are frequently desired to be produced by the very feeble currents used for signalling or for the transmission of speech. The discovery and development of permalloy, therefore, promises enormous assistance to the communication art because of its ease of magnetization by the magnetic fields produced by feeble currents. In submarine cables of practical dimensions greater speed of signalling can be obtained if the signalling currents can be caused to produce in the surrounding space a suitably greater magnetic flux, that is, if the distributed inductance of the cable can be suitably raised. An increase in speed of signalling from the ordinary limit of about 250 to over 1900 letters per minute was attained in the case of the New York-Azores cable by designing both the cable and its terminal apparatus to take advantage of increased inductance of the former obtained by magnetically loading it with permalloy. The central conductor of the cable was wrapped with a thin tape of permalloy and thus surrounded by a medium in which the feeble signalling currents produce relatively enormous changes in magnetic induction.
