Strong Fields at Different Temperatures. 295 



magnetization, i. e. moment per unit mass ; 0, temperature ; 

 6 , rotation of plane of polarization ; K, " Kerr's constant/' 

 C.Gr.S. units are tacitly assumed, wherever the contrary is not 

 expressly stated ; compare loc. cit. § 5. 



I. Magnetometric Observations. 



§ 3. Apparatus. — Four elongated ovoids (prolate ellipsoids 

 of revolution) of length 18 centim. and diameter '6 centim. 

 (ratio 1/30) were prepared and could be uniformly magnetized 

 in the coil. On account of their non-endlessness a correction 

 of *052 3 * had to be subtracted from the field $ of the coil 

 in order to obtain the real magnetic intensity «£/ inside the 

 ovoid. The following metals were tested : — 



(a) Soft Swedish iron, carefully annealed : D = 7'82. 



(b) Hard English cast steel, yellow-tempered at 230° : 

 D = 7-78. 



(c) Hard-drawn best nickel wire : D = 8*82 (from the 

 " Mckelwalzwerk " in Schwerte, Westphalia) ; stated by the 

 makers to contain 99 per cent. Ni, besides Si0 2 and traces of 

 Fe and Cu. 



(d) Cast cobalt : D about 8'0 (from Messrs. Johnson and 

 Matthey). My friend Dr. Serda kindly analysed this for me 

 with the following result : — Co 93*1 per cent., Ni 5*8 per 

 cent., Fe "8 per cent., Cu *2 per cent., Si *1 per cent., C '3 

 per cent. Rather unsatisfactory brittle material ; the ovoid 

 broke in the lathe ; it was thought best to scrape the frac- 

 tured ends truly plane, and to hold them tightly pressed toge- 

 ther in a brass tubular screw-and-nut arrangement f . 



The coil was wound to the following dimensions : length 

 30 centim.; inner diameter 4 centim.; outer diameter 12 

 centim.; 12 layers, each of 90 turns, of double cotton-covered 

 shellacked wire, '25 centim. in diameter ; resistance cold *9 

 ohm. It could be surrounded by an ice-jacket ; wdth a 

 dynamo-current of 32 amp. almost a kilo-watt of activity was 

 being dissipated in it, sufficient to melt a kg. of ice every five 

 minutes. In other cases the coil was electrically heated. 



§ 4. The ovoids, fixed inside the coil, could be maintained 

 at 0° or 100° by ice or steam properly applied. The arrange- 

 ment for heating them over 100° and the mode of deter- 

 mining the field are described, loc. cit. §§4 and 14. The 

 whole of this apparatus was placed in Gauss's " first position," 

 about 135 centim. due east of a magnetometer. It was 



• See Maxwell, Treatise, 2nd ed. ii. § 438, form. (14). 

 t See J. J. Thomson and Newall, Proc. Cambr. Phil. Soc. vi. p. 84 

 (1887) ; Ewing and Low, Phil. Mao-. [5] XXY i. p> 975 (1888). 



2 A2 



