OK THE MAGKBTIO QUALITIES OF IRON, 
1035 
considerable section, especially when the material is soft iron, unless the ballistic 
galvanometer has a much longer period than is usual. The need of insulation, as well as 
lamination, is well exemplified by the case of Bing VIIL, described above at p. 1008, 
Experiments with Groups of Pivotted Magnets, 
The remainder of our experiments relate to the molecular theory of magnetic 
induction discussed in a former paper by one of us,* It was there shown that, if we 
adopt Weber’s view that the molecules of a magnetizable metal possess magnetic 
polarity, the phenomena of hysteresis and other prominent characteristics of the 
magnetizing process follow as consequences of the control which the molecular magnets 
exert on one another by their polar forces. Thus, in the application of magnetic force 
to an unmagnetized piece, the molecular magnets are at first deflected stably, and in 
this stage there is no hysteresis. But when the force is sufliciently increased certain 
molecules are deflected so far as to become unstable, and a breaking up and re- ' 
arrangement of molecular groups supervenes. Finally, after this process has occurred 
throughout the piece a third stage is entered on, when further stable deflections 
through a small range bring about the completely “ saturated ” state. Hysteresis is 
found whenever any of the molecules pass from one stable conflguration to another 
through a phase of instability, and this transition, being mechanically irreversible, 
involves a dissipation of energy. It was shown that this conception of molecular 
structure gave an explanation of very many known facts regarding the magnetic 
quality of iron and steel, and that some of these may be effectively illustrated by 
means of a model consisting of a group of small magnets pivotted like compass needles 
on fixed centres, and placed near enough to one another to allow their mutual polar 
forces to be strongly felt. 
In further elucidation of this theory, we have made a number of experiments with 
such a model. In some of them as many as 130 little magnets have been used. These 
were cut out of sheet steel. They wmre mostly lozenge-shaped, and were about an 
inch long and half an inch wide, but some were smaller; each had a central recess 
punched in it to form a socket for the supporting pivot, which was an inverted 
drawing-pin with a sharp point. The pins were stuck through a sheet of paper to 
hold them at the desired distance apart, and the paper was laid on a horizontal 
sheet of glass which stood between a pair of coils, through which current was passed 
to produce a deflecting magnetic field. Two layers of such magnets were generally 
used, one above the other. The aggregate magnetic moment of the group was 
measured by means of a mirror magnetometer, which w^as furnished with an adjustable 
compensating coil to neutralize the direct effect of the current in the two field-coils of 
the model. 
The group having been well stirred up and allowed to settle in the absence of 
* “ Contributions to the Molecular Theory of Induced Magnetism,” ‘ Roy. Soc. Proc.,’ vol. 48, p. 342 
(.Tune, 1890). 
6 Q 2 
