Table 444. ■351; 



MAGNETIC PROPERTIES. 



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

 47r lines of force radiate from it. M, pole strength; ^irM 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. 



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

 section of uniformly magnetized pole face, and V is the volume of the magnet. /^irM/A = 4x7 = 

 no. lines of force leaving unit area of pole. 



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



4>, magnetic flux, = 47rM + HA for magnet placed in field of strength H (axis parallel to field). 

 Unit, the maxwell. 



B, flux density (magnetic) induction, = <p/A = 4x7 + H; unit the gauss, maxwell per cm. 



/I, magnetic permeability, = B/H. Strength of field in air-fiUed solenoid = H = (47r/io) 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; fi = 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 7, then the ratio I/H = ((JL — i)/4 iris 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. 

 fi = 47r/c + 1. 



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



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



7^, /ji, similarly atomic and molecular intensity of magnetization. 



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

 = SH dl = {1/ 4Tr) J'H dB. Steinmetz's empirical formula gives a close approximation to the 

 hysteresis loss; it is aB^'^ where B is the max. induction and o is a constant (see Table 472). The 

 retentivity (Br) 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) . 



Ferromagnetic substances, jj. 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. ju for 

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



Paramagnetic substances, fi>i, very small but positive, K = lo"* to io~*: oxygen, especially 

 at low temperatures, salts of Fe, Ni, Mn, many metallic elements. (See Table 474.) 



Diamagnetic substances, n<i, k negative. Most diamagnetic substance known is Bi, —14 

 X io~*. Volume susceptibility (see Table 474). 



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 487 and 488. 



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 aUoys show first a small increment Al/l = 

 (7 to 35) X 10""^, then a decrement, and for H = 1600, Al/l may amount to -(6 to 8) x io~*. 

 Cast cobalt with increasing field first decreases, Al/l = —8 Xio~®, 77 = 150, then increases in 

 length, Al/l = + $ X io~*, H = 2000; annealed cobalt steadily contracts, Al/l = -25 x io~*, H 

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

 -36 X io~*, H = 2000 (Williams, Phys. Rev. 34, 44, 1912). 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, 1911). A reciprocal effect is observed in that when a rod of soft 

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



Villari effect; reaUy 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. 



