Chap. 8] 



MAGNETIC METHOD 



303 



The effect of remanent magnetization is eliminated by observation in 

 two positions. The apparatus may be calibrated with iron chloride solu- 

 tion whose magnetic susceptibiUty k = 6(88. 78p — 0.78), where 5 = den- 

 sity, and p = percentage of iron chloride. In a modification of this 

 method, a cast is made of the specimen from a material of known sus- 

 ceptibility, and the comparative effects of sample and cast are measured 

 on an astatic magnetometer.^ By the use of an astatic system, variations 

 of the earth's magnetic field are eliminated. This method has the ad- 

 vantage of being independent of the shape of the sample. 



In the following procedures, suscepti- 

 bility determinations are made in variable 

 magnetic fields greater than that of the 

 earth. These determinations make it pos- 

 sible to trace a complete hysteresis curve. 

 The earliest of these made use of the 

 Coulomb torsion balance (Wiedemann, 

 Curie-Ch^n^veau, Wilson). The specimen 

 (enclosed in pulverized form, in a small 

 vial) is attached to one end of the balance 

 and exposed to the attraction of a strong 

 electromagnet whose magnetic moment is 

 determined by means of a steel mirror 



-y./y J '^,M// /yy/M \'//y//////////////////M. 



fmaqe 



Po/e 



Fig. 8-4. Arrangement of 

 specimen and vertical magnet- 

 ometer system, for susceptibility 

 determination. 



placed at a distance of about 

 2.5 m. Since the magnetic mo- 

 ment of the specimen is kHv and 

 the torque on a magnet of the 

 moment M in a magnetic field H 

 is AfH, the couple acting on the 

 torsion balance is proportional to 

 the square of the field and hence 

 to the square of the current 

 in the electromagnet. In the 

 Curie-Ch4n6veau and Wilson bal- 

 ances, the specimen is drawn into the gap of a permanent ring-shaped 

 magnet.* 



Very similar in application is the balance method of Lord Kelvin, which 

 uses a cyUndrical specimen suspended from one end of a scale with its 

 lower end between the poles of a strong electromagnet (see Fig. 8-5). 

 The specimen is first weighed. After current is turned on, balance 

 is re-established by additional weights. If the force of attraction 



Fig. 



8-5. Balance method of susceptibility 

 determination (after Haalck). 



^See C. A. Heiland, A.I.M.E. Geophysical Prospecting, 234-236 (1932). 

 » For details see P. Curie and C. Ch6n6veau, Journ. de phys., 4(2), 796 (1903); 

 Comptes Rendus, 160, 1317 (1910); Phil. Mag., 20, 257 (1910). 



