

Magnetic Permeability, $c., of Iron at Low Temperatures. 93 



magnetism by the application of an alternating current, the iron is so 

 altered in magnetic qualities that it is impossible to repeat two sets 

 of observations under precicely similar circumstances. With regard 

 to the unannealed iron, it may be noted that if an ordinary magnet- 

 isation curve is taken up to very high magnetisation forces, and the 

 iron then demagnetised by the application of an alternating current 

 gradually reduced, the first magnetisation curve can never be 

 repeated exactly again on applying increasing magnetisation forces, 

 but a curve is obtained which lies slightly inside the first curve, and 

 which indicates that the permeability has been reduced. The sub- 

 sequent repetition of this process will give a series of curves which 

 occupy different positions, but which do not precisely repeat any of 

 them. Hence it is impossible to repeat at a constant temperature with 

 this unannealed iron exactly any magnetisation or permeability curve. 

 In the case of the annealed iron it is quite different. A. magnet- 

 isation curve can be obtained after having carefully de-magnetised 

 the iron, if this magnetisation is pressed up to nearly its limit and 

 the iron then de-magnetised by the application of an alternating 

 and decaying magnetising force, a second magnetisation curve can be 

 obtained on again applying an ascending magnetising force, but it 

 will not coincide exactly with the first curve. The annealed iron 

 can, however, be brought back into its original condition by dipping 

 it a few times into liquid air. Under these conditions, we have been 

 able to repeat as frequently as required the observations with the 

 annealed iron taken at the different temperatures. In the case of the 

 unannealed iron the changes produced in it by immersing it in the 

 liquid air and by magnetising and demagnetising it, are such as to 

 render it almost impossible to obtain results capable of precise 

 repetition, with respect to the hysteresis loss and permeability for 

 varying magnetising forces. 



Experiments with Hardened Iron. 



A third set of experiments were taken with a ring coil of the same 

 dimensions as the ring coil made of soft annealed transformer iron 

 first described. This third coil was constructed of the same sample 

 of Sankey's transformer sheet iron as the above described soft 

 annealed ring, but it was treated subsequently to its formation in 

 the following manner : 



A short piece of iron gas-pipe was made red hot in a forge ; the 

 ring coil, having been constructed, was dropped into the red-hot pipe, 

 and the ends of this pipe loosely plugged up with slag wool ; the red- 

 hot pipe was then covered ever with cinders, and the mass allowed to 

 cool. Under these conditions the ring coil was annealed in an atmo- 

 sphere of carbonic oxide and in contact with hot carbon ; the sheet 



