GENERAL ACCOUNT OF MAGNETIC ACTIONS 187 



tions of temperature, to get rid of a sort of temperature hysteresis 

 which occurs at first, then subsequent rise of temperature within 

 ordinary range results in diminution of magnetisation, but on the 

 fall of temperature to the original value the magnetisation increases 

 to its original value. This may be easily verified by an experiment 

 represented in plan in Fig. 134. A needle, N S, with an attached 

 mirror is suspended so as to throw an image on a scale. Two 



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IS NJ -W- IN si 



""' y Cool 



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I HJ. 134. 



magnets are brought up, one on either side, at such distances that 

 t lit-y neutralise each other's action on the needle. They are fixed in 

 position. Then one magnet is warmed, say, by bringing up from 

 below a beaker of warm water to surround it. At once its 

 magnetisation is diminished, and the needle shows by its deflec- 

 tion that the cool magnet is the stronger. When the temperature 

 is reduced once more to its original value the needle returns to its 

 original position. By this arrangement the magnet whose tempera- 

 ture is varied is brought quite near to the needle, so that a small 

 change in its magnetisation may produce a considerable deflection. 

 K wing* found that if a bar is only very weakly magnetised, then 

 rise of temperature produces increase of magnetisation. As the 

 bar is more strongly magnetised this increase falls off and ultimately 

 changes sign. In ordinary steel bar magnets the effect is always 

 decrease in strength with rise of temperature. 



Permeability and temperature. A large number of 

 experiments have been made with different qualities of iron and 

 steel to investigate the change of permeability with change of 

 temperature.")" We shall here only describe the results of some 

 experiments of Hopkinson on a specimen of Whitworth mild steel. 



* .!/./'//"//' Iil <-t ;,,ii, in Iron and other Metals, 3rd ed. p. 181. 



t Kwing, loc. cit. chap, viii, gives an account of the fundamental work on the 

 effects of change of temperature. A paper by S. P. Thompson on The Magnetism 

 of Pennant ut .)/"<//" /x (Jonrn. Inxt . Elect. En<j. y vol. 1. p. 150, 1913) gives an excellent 

 account of the properties of permanent magnets and their dependence on the 

 nature of the steel from which they are made. The effect of temperature on 

 magnetisation is described. A bibliography is given. 



