RESEARCHES IN MAGNETISM. 
527 
frictional retardation (resembling the friction of solids), which must be overcome by 
the magnetising- force before deflection begins at all. Again, as the magnetising force 
begins to be withdrawn, this friction must be overcome by the quasi-elastic restoring 
force before each molecule can begin to return towards its primitive position ; and 
the molecule finally takes up a position determined by the equilibrium of the 
restoring force and the frictional resistance to further return. It seems even possible 
that the residual magnetism which is found in soft iron when an applied magnetising 
force is completely removed, may be due entirely to this frictional sticking of the 
molecules. Hard iron and steel, on the other hand, in which the residual magnetism 
is more permanent, behave in a way which suggests the combination of this frictional 
sticking with something else of the nature of Maxwell’s permanent set. 
The frictional resistance must resemble the friction of solids, not the viscosity of 
liquids, since it appears to be independent of the speed at which the changes of 
magnetic condition occur; and since the magnetisation which any field produces is 
reached almost at once on the application of the field, and, so long as the field is kept 
constant, suffers little or no change with lapse of time. 
§ 6. The following notation, adopted from Maxwell, will be used throughout this 
paper :— 
tip is the magnetising force. 
23 is the magnetic induction within the metal. It is the number of “lines of 
force ” per square centimetre which would be found if we were to cut an 
indefinitely narrow crevasse perpendicular to the direction of magnetisation. 
3 is the intensity of magnetism, or the magnetic moment of the metal per cubic 
centimetre. 
is the coefficient of induced magnetisation. 
23 
p-, is the magnetic permeability, or magnetic inductive capacity. 
53 — 4 7r2> + «§ 
li — ^TTK + 1 
The values of 23, and 3 will be given in c.g.s. units. 
In representing graphically the results of experiments, I have generally plotted 
either © or 3 in terms of This is the most direct and, for most purposes, the 
most useful mode of representation. Another plan, used with good effect by 
Rowland, is to plot either k or p in terms of 3 or 23, and this has been adopted 
in a few instances. 
§ 7. When a bar of finite length is magnetised by the action of an electric current 
in a surrounding solenoid, the magnetising field is due partly to the solenoid and 
partly to the ends of the magnet itself. During magnetisation the field due to the 
ends of the magnet opposes the action of the solenoid and reduces the resultant field, 
3 Y 2 
