1896.] 



on Electric Besearch at Low Temperatures. 



265 



ring, which has two coils of wire wound round it, and connect one of 

 these circuits to a battery, we shall send a current through this 

 primary coil and magnetise the iron ring. If the other, or secondary 

 circuit is connected to a galvanometer, then at the instant of starting 

 the primary current there is a transitory induced current produced 

 in the secondary circuit. As long as the induction in the iron 

 remains constant no electric change will take place in this secondary 

 circuit. If, however, we plunge the 

 iron ring into liquid air, whilst 

 still keeping the primary current 

 constant, we find again a secon- 

 dary current produced at the 

 moment of cooling the iron. 

 This indicates a sudden increase 

 of permeability at the instant of 

 cooling. If we bring the ring 

 out of the liquid air we find it 

 retains some of the increased 

 permeability acquired on cooling, 

 but loses a portion of it more 

 slowly if it is heated up again 

 to ordinary temperatures by 

 plunging it into a bath of alco- 

 hol. Owing to these changes 

 we found it impossible to repeat 

 again exactly any required mag- 

 netisation curve in the case of 

 the hard iron. The sudden cool- 

 ing alters the magnetic qualities 

 of the unannealed iron to such 

 an extent that it is not possible 

 to get it twice in exactly the 

 same state. 



By subjecting a hard iron 

 ring to frequent reversals of the 

 same magnetising force, whilst it 

 is warmed up slowly from the 

 temperature of liquid air up to 

 ordinary temperatures, we have 

 been able to trace the gradual 

 decrease of the permeability at 

 any constant force throughout this range of temperature, and the 

 results are embodied in the series of curves in Fig. 18. 



We have found, on the other hand, that unhardened steel pianoforte 

 wire behaves like soft annealed iron. 



We have then examined the hysteresis of iron at low temperatures. 

 As the meaning of that term was very fully explained by the inventor 

 of it in a discourse given quite recently, no time need be spent in an 



TEMPERATURE IN PLATINUM DEGREES. 

 -200° -150° -100° -50° 0° 



Fig. 18. 



Curves showing the variation of per- 

 meability of iron with temperature 

 between 0° C. and -200° C. 



