ISO Loomis — Effects of Changes of Temperature on 



The temperature limits were 0° and 80° R. By swinging 

 their magnets at different temperatures they found the change 

 in moment proportional to the difference in temperature, as is 

 shown by their formula. 



Prof. G. Wiedemann* lias made some careful investigations 

 on the influence exerted by the temper of the steel and the 

 original intensity of magnetization. He used bars 22 cm long 

 and l*35 cm in diameter. Before they were magnetized these 

 bars were placed. alternately in melting snow and boiling water 

 fifteen times, in order to bring the steel itself as far as possible 

 into such a state that alterations in temperature would produce 

 no structural change. The bars were magnetized in a coil at 

 a temperature of 0° C. They were then carefully placed in a 

 box of sheet copper before the needle of a magnetometer and 

 the deflection was observed by telescope and scale. The tem- 

 peratures of 0° and 100° C. were obtained by means of melt- 

 ing snow and boiling water. His results for magnets that have 

 reached the permanent state show that in case of hard steel 

 magnets the change in moment is nearly proportional to the 

 moment at 0° C, while for tempered and soft steel magnets, 

 the ratio of change to the moment at 0° C. increases with the 

 moment. As his results give a good idea of the size of the 

 changes under discussion, I append the following table from 

 his paper. 



With reference to the theory of these changes Prof. Wiede- 

 mann says: " Besides the permanent effect due to an alteration 

 in temperature there is a temporary change. Each heating 

 diminishes the permanent moment of the molecules. More- 

 over, for the time being, it loosens the particles of the body 

 and lessens the strain in which they have been placed by the 

 action of external forces, therefore they return a little toward 

 their first position of equilibrium, in which they were held by 

 the forces acting between them before the external forces came 

 into play. Heating thus diminishes the magnetization tempo- 

 rarily ; but, on cooling, the molecules return to their former 

 position and the lost magnetization is regained. We can 

 produce entirely analogous phenomena if we change the tem- 

 perature of bodies which have suffered a change of form (tor- 

 sion) as a result of mechanical forces, and observe the increase 

 and decrease of this on heating and cooling." 



Barus and Stronhalf carefully distinguished the mechanical 

 effect of heating from the purely magnetic effect. They found 

 that a temperature of 20° or 30° C. above that of the water in 

 which a glass-hard steel rod was dipped in hardening produced 

 quite perceptible annealing effects. This change in the hard- 



*G. Wiedemann, Pogg. Ann., c, p. 235, 1852; ciii, p. 563, 1858; cxxii, 

 p. 355, 1864. f Bull. U. S. G. S., No. 14, p. 151. 



