Permalloys

Figure 6. Permeability curves for Permalloys. (Reference 4.)

Powdered Iron.^7,8,9 Special powdered iron cores have been used in radio-frequency coils and transformers since about 1935. Substances commonly used are⁷ hydrogen-reduced iron, carbonyl iron, and magnetic iron oxide, commonly known as magnetite (Table III). These cores are usually molded "slugs" of finely divided iron, each particle of which is insulated and held by a suitable binder. In many radio coils and transformers the magnetic core is mounted on a screw so that the core may be moved inside the coil to vary the inductance. These coils are smaller, use less wire, have a higher Q, and are less expensive than comparable air-core coils.

Materials for Permanent Magnets.¹⁰ Steels of high carbon content were used extensively for permanent magnets for many years. Later, cobalt steel, having better characteristics, was developed. More recently, alloys (such as Alnico) of aluminum, nickel, iron, and other elements, such as cobalt, have been used for permanent magnets.

Figure 7. Hysteresis loops of silicon steel and Permalloy. On the same scale, the loop for Perminvar would be a straight line.

TABLE I

DESIGNATIONS AND COMPOSITIONS OF SOME COMMUNICATION MAGNETIC ALLOYS

(From reference 4,)

Designation Composition, Per Cent

A material called Vicalloy composed of vanadium, iron, and cobalt was announced¹¹ in 1940. The characteristics of certain of these materials are shown in Fig. 8.

TABLE II

MAGNETIC CONSTANTS FOR COMMUNICATION MAGNETIC ALLOYS (From reference 4.)

Here μ₀ and μ_m are the initial and maximum permeabilities, respectively; W_H=inf is the hysteresis loss in ergs per cubic centimeter per cycle for saturation value of flux density; B_r is the residual induction in gausses; H_c is the coercive force in oersteds; (B - H)_H=inf., is the saturation value of the intrinsic induction in gausses; ρ is the resistivity in microhms-centimeter.

TABLE III

CHARACTERISTICS OF CORE MATERIALS USED IN RADIO COILS (From reference 9.)

Eddy-Current Core Hysteresis Loss Residual Loss Loss

Coefficient Coefficient Coefficient

Quantities Units

μa 10^-3 ohms/henry, cps, gauss

a 10^-6 ohms/henry, cps, gauss and per unit permeability

μc 10^-2 ohms/henry, cps

c 10^-5 ohms/henry, cps and per unit permeability

μe 10^-7 ohms/henry, cps²

e 10^-9 ohms/henry, cps² and per unit permeability

(*) 40-L is a core made of Carbonyl iron, type L with a permeability of 39. (+) 40-H is a core made of one of the best grades of commercial hydrogen-reduced iron, with a permeability of 42.

(++) Carbonyl iron type E with a permeability of 16.5.

Miscellaneous Magnetic Materials. Several other interesting magnetic materials have been developed for telephone purposes. Among these is Perminvar¹² listed in Table I. The permeability of this material is constant over a wide range of low flux densities, and the hysteresis loss is very low, even compared with Permalloy. Another group of alloys, known as Permendurs, have high permeability over a wide range of high flux densities.

Figure 8. Characteristics of certain permanent magnet materials. A large demagnetizing force required to reduce the flux to zero indicates a superior material. (Data from Reference 10.) A. Nickel-aluminum-iron alloy. C. 35% Cobalt-steel alloy. B. Cobalt-molybdenum-iron alloy. D. Cobalt-tungsten-iron alloy. E. Chromium-steel alloy.