32 THE QUANTUM [ III>3 
behaviour in a magnetic field is analogous to that of conducting 
spheres in an electric field. 
The real founder of the molecular theory is Weber,* who 
assumed that the molecules of an electric substance are them 
selves permanent magnets. Clerk Maxwell has said : “ We 
may think of every portion of the magnet as polarized ; that is 
to say, every particle or elementary piece of the bar may be 
regarded as a separate magnet.” “ Magnetization is a pheno 
menon, not of large masses of iron, but of molecules ; that is 
to say, of portions of the substance so small that we cannot by 
any mechanical method cut one of them in two, so as to obtain 
a north pole separate from a south pole.” “ There are strong 
reasons for believing that the act of magnetizing iron or steel 
does not consist in imparting magnetization to the molecules 
of which it is composed, but that these molecules are already 
magnetic, even in unmagnetized iron, and that the act of 
magnetization consists in turning the molecules so that their 
axes are either rendered all parallel to one direction, or at least 
are deflected towards that direction.” 
The second essential feature of the modern theory was also 
realized by Weber, as has been pointed out by W. Peddie.f 
In the words of Clerk Maxwell : “ The molecules do not turn with 
their axes parallel to % (the direction of the magnetizing force), 
and this is because each molecule is acted on by a force tending 
to preserve it in its original direction, or because an equivalent 
effect is produced by the mutual action of the entire system of 
molecules.” Weber postulated a simple expression for the 
magnitude of the internal field spoken of in the last clause, 
saying : “ But this back-acting force, arising from the mutual 
actions of the molecules, must increase with the deflection and 
can be represented by D sin p, where D denotes a constant 
magnitude which one can call the molecular directive force.” 
According to the hypothesis of Ampère, the magnetization 
of the molecule is due to an electric current circulating in some 
closed path within it. As these molecular currents must con 
tinue without diminution it is necessary to suppose that there 
is no resistance offered to the passage of the current. Ampère’s 
theory was devised before the induction of currents was known. 
Weber extended the theory and explained the phenomenon of 
diamagnetism as due to currents induced in the molecules by a 
variation in the magnetic field. These “molecular magnets” 
are now frequently spoken of as the “ Weber elements,” a term 
which is preferable because the former term suggests that the 
molecular magnet corresponds to the chemical molecule, which 
Weber, Theory of Magnetism,” Pogg. Ann., voi. 87, p. 167, 1852. 
t W. Peddie, Nature, voi. 120, p. 80, 1927.