182 Loo mis — Effects of Changes of Temperature on 



the distribution of magnetism. — Knpfer* determined at two 

 different temperatures the period of vibration of a short needle 

 placed opposite different parts of a long magnet and found the 

 proportional change in distribution greater at the ends than in 

 the middle of the bar. All his measurements were made before 

 the bar had reached the permanent state. 



Polonif measured the distribution at various temperatures, 

 by slipping a coil from different parts of the magnet to such a 

 distance that the magnet exerted practically no effect and 

 measuring the quantity of electricity thus induced. He worked 

 between the temperatures 0° and 200° C, using an oil bath to 

 obtain his high temperatures. The changes were quite regu- 

 lar between 0° and 180° C, but were very large near 190° C. 

 Between and 180 C, he found that the following formula held : 



M = A [1 + kr 4 - h~* - *w>] 

 in which M is the induction in the magnet at a distance x from 

 one of the ends ; Z, the length of the magnet ; A, a quantity 

 depending only on the temperature, while k is sensibly con- 

 stant for a given magnet. 



II. Experiments and Calculation of Results. 



The existence of a permanent state, in which the moment 

 of a magnet increases or diminishes as its temperature falls or 

 rises, being now well established, the reason for this change 

 becomes an interesting subject for investigation. In the hope 

 of obtaining some clue to its real nature, the following experi- 

 ments were undertaken. They were planned to determine: 

 first, the change in magnetic moment due to change in tem- 

 perature in bars of the same cross-section but of different 

 lengths; second, the change in distribution due to change in 

 temperature. The experiments will be considered in the 

 above order. 



A, Experiments on the change of the magnetic moment of 

 magnets of different lengths but of the same cross- section. — 

 Stubb's steel wire of square cross-section, 0'159 cm square, was 

 cut into lengths of approximately 5*5 cm , 8'3 cm , and 22 cm . The 

 steel was soft and was used of the same temper as purchased. 

 The bars were annealed in boiling water, magnetized to satu- 

 ration in a coil, and were in the permanent state when used. 

 The period of vibration in the earth's field was determined at 

 11° and 99° C. A double box of thin sheet zinc was used to 

 keep the magnets at the required temperatures. In the top of 

 the box was a round opening into the interior, in which a cork 

 holding a thermometer and a glass tube was inserted. Through 

 this tube passed a short wire, which supported the magnet and 

 was suspended from the ceiling by cacoon silk. The suspen- 



* Kupfer, Pogg. Ann., xii, p. 133. 



t Poloni, Beibl. v, 802. Atti della R. Acad, dei Lincei, v, p. 262, 1881. 



