60 



Prof. J. A. Ewing. 



[Jan. 8, 



electromagnets must be due chiefly to other causes than this static 

 hysteresis, and is, in fact, due almost wholly to the induction of so- 

 called Foucault currents in the cores. The relation of this hysteresis 

 to Weber's theory of molecular magnets, as extended by Maxwell, is 

 discussed, and the insufficiency of Maxwell's extension noticed. 



By vibrating a piece of soft iron during the application and 

 removal of magnetising force, the effects of hysteresis are almost entirely 

 removed, and the iron is then found to possess almost no retentiveness. 

 But when the application and removal of magnetising force are 

 effected without mechanical disturbance, the retentiveness of soft 

 iron is found to be even greater than that of steel. In some cases 

 93 per cent, of the whole induced magnetism of a piece of annealed 

 iron, was found to remain, on the complete removal of the magnetising 

 force. It is pointed out that there is no discrepancy between this 

 result and the well-known fact that a short iron core of an electro- 

 magnet retains almost no magnetism when the current in the magnet 

 is interrupted. In that case the ends of the magnet itself, after the 

 interruption of the current, exert a sufficient reversed magnetising 

 force to destroy almost entirely the residual magnetism. But when 

 tested under the conditions which give uniform magnetisation and 

 avoid the demagnetising influence of the ends, soft annealed iron is 

 more retentive than even the hardest steel. 



Examples are given showing that the influence of permanent set in 

 the curve of magnetisation is so marked as to give a criterion by 

 which a strained piece may be readily distinguished from an annealed 

 piece of metal, and that strain diminishes very greatly the magnetic 

 retentiveness of iron. 



Numerical values of the coefficients of permeability (ju) and of sus- 

 ceptibility (k) are given for a number of samples of iron and steel, 

 and the relation of these coefficients to J and |( is exhibited 

 graphically after the manner of Rowland. The greatest value of p 

 refers to soft annealed iron while under mechanical vibration, and is 

 about 20,000. 



The next part of the paper deals with the effects of stress (consist- 

 ing of longitudinal pull ) on the magnetic susceptibility and retentive- 

 ness of iron, and the subject is taken up in two different ways. In 

 one the magnetic effects are observed when a load hanging from the 

 wire under examination is gradually applied, removed, and otherwise 

 varied, the wire being then either kept in a magnetising field of 

 constant value, or freed from all magnetising force. In the other 

 method the load is kept constant, and magnetising force is applied, 

 removed, and otherwise varied. The two methods give results which 

 would be identical were it not for (1) hysteresis in the relation of 

 magnetism to load, in a constant field, and (2) hysteresis in the rela- 

 tion of magnetism to field, under constant load. The relation of mag- 



