134 



Prof. J. A. Ewing. 



transient currents to change if the magnetising force were sufficiently 

 increased. Recently I have examined the effects with very high 

 values of the longitudinal magnetising force. The transient currents 

 are then exceedingly small, but they obstinately refuse to become 

 reversed, even when the magnetising force is as much as 100 c.g.s. units. 

 The effects still correspond to greater magnetisation along the lines of 

 pull. 



The explanation of this seeming anomaly lies in the fact that the 

 Villari reversal of the effects of stress depends on the intensity of 

 magnetisation rather than on the value of the magnetising force, and 

 that the stress acts here only on a component of the whole magneti- 

 sation. 1 find that in soft iron, the effect of a moderate amount of 

 pull is to give greater susceptibility so long as the magnetisation does 

 not exceed about three-fourths of its limiting value. With a greater 

 stress the reversal comes earlier, and with a very small stress it 

 comes later. Now, suppose that we apply to a soft-iron wire a 

 powerful magnetising force and thereby approach the limit of mag- 

 netisation. When we twist the wire, the directions of pull and push 

 are inclined at 45° to the direction of magnetisation, and the stress 



therefore acts on about seven-tenths of the whole intensity of 



magnetisation. Hence pull ought still to increase (though very 

 slightly) the magnetic susceptibility along the lines of pull, and the 

 circular magnetisation ought still to have the same direction as it had 

 when the magnetising force was small; and this is what actually 

 occurs. 



In the paper, it was noticed that when the magnetising current 

 was interrupted while the wire was held in a constant state of twist, 

 there was scarcely any change of the circular magnetisation. I now 

 find that although in general the circular magnetisation decreases very 

 slightly in these conditions, nevertheless it occasionally increases when 

 the longitudinal magnetising force is removed. This happens in soft iron 

 when that force is very high, and its occurrence is in complete agree- 

 ment with direct observations of the effects of pull on magnetism. 

 Jt means that the effect of pull is then greater on the 45° component 

 of the residual magnetism than on the same component of the tem- 

 porary magnetism, and this is the case when the affected component 

 of the temporary magnetism is near that value at which the Villari 

 reversal occurs. The following figures give an observed instance of 

 this action in a very strougly magnetised soft-iron wire which was 

 held in a constant state of twist. Calling the two directions of the 

 magnetising current A and B, and the two directions of circular 

 magnetisation + and — , we have : 



