Table 549 463 



MAGNETIC PROPERTIES 

 DEFINITIONS AND GENERAL DISCUSSION 



Unit pole is a quantity of magnetism repelling another unit pole with a force of one dyne; 

 4T lines of force radiate from it. M , pole strength; 4irM lines of force radiate from pole of 

 strength M. 



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

 one line per unit area. 



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



/, intensity of magnetization or pole strength per unit area, = 9A/V = M/A where A is cross 

 section of uniformly magnetized pole face, and V is the volume of the magnet. 4W1W/A = \irl = 

 no. lines of force leaving unit area of pole. 



J, specific intensity of magnetism, = I/p where p = density, g/cm 3 . 



<f>, magnetic flux, = ^.ttM + HA for magnet placed in field of strength H (axis parallel to field). 

 Unit, the maxwell. 



B, flux density (magnetic) induction, = <f>/A = \irl + H; unit the gauss, maxwell per cm. 



H, magnetic permeability, = B/H. Strength of field in air-filled solenoid - H = fair /id) ni 

 in gausses, i in amperes, >i, 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; jj. = B/H. 



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 = (jj. — i)/4iris the magnetic susceptibility per unit volume 

 and is a measure of the magnetizing effect of a magnetic field on the material placed in the field. 



/JL = 47TK +1. 



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



Xa> atomic susceptibility, = X X (atomic weight); Xm = molecular susceptibility. 



J A , 7 M , similarly atomic and molecular intensity of magnetization. 



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

 = J*H dl = (i/4ir)J'H dB. Steinmetz's empirical formula gives a close approximation to the 

 hysteresis loss; it is aB 1 ' 6 where B is the max. induction and a is a constant (see Table 5/5). 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 (H c ). 



Ferromagnetic substances, /j. very large, k very large: Fe, Ni, Co, Heusler's alloy (Cu 62.5, 

 Mn 23.5, Al 14. See Stephenson, Phys. Rev. 1910), magnetite and a few alloys of Mn. p. for 

 Heusler's alloy, 90 to 100 for B = 2200; for Si sheet steel 350 to 5300. 



Paramagnetic substances, fx> 1, very small but positive, K = io -3 to io -6 : oxygen, especially 

 at low temperatures, salts of Fe, Ni, Mn, many metallic elements. (See Table 580.) 



Diamagnetic substances, /jl<i, k negative. Most diamagnetic substance known is Bi, -14 

 X io -6 . Volume susceptibility (see Table 580). 



Paramagnetic substances show no retentivity or hysteresis effect. Susceptibility independent 

 of field strength. The specific susceptibility for both para- and diamagnetic substances is in- 

 dependent of field strength. 



For Hall effect (galvanomagnetic difference of potential), Ettinghausen effect (galvanomagnetic 

 difference of temperature), Nernst effect (thermomagnetic difference of potential) and the Leduc 

 effect (thermomagnetic difference of temperature), see Tables 593 and 594. 



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 increment Al/l = 

 (7 to 35) X IO_7 > tnen a decrement, and for H = 1600, Al/l may amount to -(6 to 8) X io -6 . 

 Cast cobalt with increasing field first decreases, Al/l = —8 Xio -6 , H = 150, then increases in 

 length, Al/l = + 5 X io -6 , H = 2000; annealed cobalt steadily contracts, Al/l = —25 x io -6 , H 

 = 2000. Ni rapidly then slowly contracts, Al/l = —30 x io -6 , II = 100; —35 X io -6 , H = 300; 

 —36 x io -6 , H = 2000 (Williams, Phys. Rev. 34, 44, 191 2). A transverse field generally gives 

 a reciprocal effect. 



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 circumstances (see 

 Williams, Phys. Rev. 32, 281, 191 1). A reciprocal effect is observed in that when a rod of soft 

 iron, exposed to longitudinal magnetizing force, is twisted, its magnetism is reduced. 



Yillari effect; really a reciprocal Joule effect. The susceptibility of an iron wire is increased 

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

 value. 



Smithsonian Tables, 



