344 Prof. J. A. Ewing. Contributions to the [June 19, 



I lately commented on the fact that soft iron and other magnetic 

 metals (notably nickel under particular conditions of strain)* show 

 a remarkably close approach to instability at certain stages in the 

 magnetising or demagnetising process. f When the magnetic force 

 reaches a particular value, the rate of change of magnetism with 

 respect to change of force may become enormous. Referring to this 

 in a paper which has just been published,;}: Mr. A. E. Kennelly has 

 reverted to the idea of chains of magnetic molecules held together 

 by the inter-molecular magnetic forces, and contends that when such 

 a chain is ruptured by applying a sufficiently strong external mag- 

 netic force it will fall to pieces throughout, and the molecular 

 magnets which compose the chain will take their alignment suddenly. 

 He accordingly sketches what he calls a " chain-theory " of mag- 

 netisation and an adaptation of the theory of Hughes, in which, 

 however, he postulates an elastic resistance to the rotation of the 

 molecules in addition to the constraint afforded by their mutual 

 magnetic forces. Mr. Kennelly 's remarks are highly interesting and 

 suggestive ; but I do not think (for reasons which will appear im- 

 mediately) that the notion of closed magnetic chains can be main- 

 tained as a general account of the molecular structure of unmagne- 

 tised iron. 



I have experimented on the subject by making a model molecular 

 structure consisting of a large number of short steel bar magnets, 

 strongly magnetised, each pivoted like a compass needle upon a 

 sharp vertical centre and balanced to swing horizontally. We 

 cannot readily imitate in a model the two degrees of rotational 

 freedom possessed by actual molecular magnets, but a group of 

 magnets swinging in one plane gives a sufficiently good general idea 

 of the nature of the equilibrium which is brought about by inter- 

 molecular forces, and the manner in which that equilibrium is dis- 

 turbed when an external force is applied. The model is very easily 

 made. Each magnet is a piece of steel wire about one-tenth of an 

 inch in diameter and two inches long (fig. 1), bent in the middle to 

 bring the centre of gravity below the point of support. The hole 

 or rather recess for the pivot is made by a centre punch : the pivot 

 itself is a sewing needle fixed upright in a small base plate which is 

 punched out of a sheet of lead. The bars swing with but little 

 friction, and their pole strength is sufficient to make the mutual 

 forces quite mask the earth's directive force when they are set 

 moderately near one another. The group is arranged on a board on 

 which lines are drawn to facilitate regularity in grouping when that 



* See a paper by H. Nagaoka, ' Journal of the Science College of the University 

 of Totio,' vol. 2, 1888, p. 30 i. 



t ' Journal of the Institution of Electrical Engineers,' No. -84, 1890, pp. 38 40. 

 I ' The Electrician,' June 7th and 13th, 1890. 



