451 

 TABLES 466-494.— MAGNETIC PROPERTIES OF MATERIALS 



TABLE 466.— DEFINITIONS*, BASIC EQUATIONS, AND 

 GENERAL DISCUSSION 



B, flux density (magnetic) induction, = <t>/A = 4-irI -+- H ; unit the gauss, maxwell per 

 cm. 



Diamagnetic substances, n<l, K negative. Most diamagnetic substance known is Bi, m = 

 .9998 k = — 14X10- 6 . 



Ferromagnetic substances, y. very large, k very large : Fe, Ni, Co, Heusler's alloy (see 

 Table 476), magnetite and a few alloys of Mn. /* for Heusler's alloy, 90 to 100 for B = 

 2,200; for Si sheet steel 350 to 5,300. 



H, field strength, = No. of lines of force crossing unit area in normal direction ; unit = 

 gauss = one line per unit area. 



Hall effect (galvanomagnetic difference of potential), Ettinghausen effect (galvano- 

 magnetic difference of temperature), Nernst effect (thermomagnetic difference of poten- 

 tial) and the Leduc effect (thermomagnetic difference of temperature), see Tables 519 

 and 521. 



Hysteresis is work done in taking a cm 3 of the magnetic material through a magnetic 

 cycle = jH dl = (\/4ir)j'H dB. Steinmetz's empirical formula gives a close approximation 

 to the hysteresis loss ; it is aB ie where B is the max. induction and a is a constant (see 

 Table 482). The retentivity (B r ) is the value of B when the magnetizing force is reduced 

 to zero. The reversed field necessary to reduce the magnetism to zero is called the coercive 

 force (He). 



I , intensity of magnetization or pole strength per unit area, = M/V — m/A where A is 

 cross section of uniformly magnetized pole face, and V is the volume of the magnet. 

 4nm/A = 4nl = No. of lines of force leaving unit area of pole. 



/, specific intensity of magnetism, = I/p where p = density, g/cm s . 



J a, Jm, similarly atomic and molecular intensity of magnetization. 



k, susceptibility ; permeability relates to effect of iron core on magnetic field strength 

 of coil ; if effect be considered on iron core, which becomes a magnet of pole strength m 

 and intensity of magnetism /, then the ratio I/H = (p. — 1)/4tt is the magnetic suscepti- 

 bility per unit volume and is a measure of the magnetizing effect of a magnetic field on the 

 material placed in the field, p. = 4itk + 1. 



M, magnetic moment = w/, where / is length between poles of magnet. 



Magneto-strictive phenomena : 

 Joule effect : Mechanical change in length when specimen is subjected to a magnetic 

 field. With increasing field strength, iron and some iron alloys show first a small incre- 

 ment Al/l= (7 to 35) X 10" T , then a decrement, and for H = 1600. &1/1 may amount to 

 — (6 to 8) X 10" 6 . Cast cobalt with increasing field first decreases, A//7 = — 8 X 10' 9 , H = 

 150, then increases in length, A/// = -f- 5 X 10~ 8 , H = 2,000; annealed cobalt steadily con- 

 tracts, M/l = — 25 X 10" e , H = 2000. Ni rapidly then slowly contracts, A/// = — 30 X 

 10- 6 , H = 100 ; —35 X 10" 8 , H = 300 ; —36 X lO" 9 , H = 2,000. A transverse field generally 

 gives a reciprocal effect. 



Villari effect ; really a reciprocal Joule effect. The susceptibility of an iron wire is in- 

 creased by stretching when the magnetism is below a certain value, but diminished when 

 above that value. 



Wiedemann effect : The lower end of a vertical wire, magnetized longitudinally, when 

 a current is passed through it, if free, twists in a certain direction, depending upon circum- 

 stances. A reciprocal effect is observed in that when a rod of soft iron, exposed to longi- 

 tudinal magnetizing force, is twisted, its magnetism is reduced. 



p., magnetic permeability, = B/H. Strength of field in air-filled solenoid = H = (47r/10) 

 ni in gausses, i in amperes, n, number of turns per cm length. If iron filled, induction 

 increased, i.e., No. of lines of force per unit area, B, passing through coil is greater than 

 H;tt = B/H. 



Paramagnetic substances, m> 1, very small but positive, k = 10" 3 to 10" 8 : oxygen, espe- 

 cially at low temperatures, salts of Fe, Ni, Mn, many metallic elements. (See Table 486.) 



Paramagnetic substances show no retentivity or hysteresis effect. Susceptibility inde- 

 pendent of field strength. The specific susceptibility for both para- and diamagnetic sub- 

 stances is independent of field strength. 



<A, magnetic flux, = 4-wm 4- HA for magnet placed in field of strength H (axis parallel 

 to field). Unit, the maxwell. 



Unit pole is of such strength that it will repel another unit pole with a force of one 

 dyne; at unit distance in free space, 4ir lines of force radiate from it. m, pole strength; 

 4-n-m lines of force radiate from pole of strength m. 



X, specific susceptibility (per unit mass) = k/p = J/H. 



Xa, atomic susceptibility, = x X (atomic weight); x« = molecular susceptibility. 



* See pages 16-18. 



SMITHSONIAN PHYSICAL TABLES 



