ATOMICITY IN ELECTRICITY
33
^to that
of cm
t inc ^:
Piassi
^et as m,'j'
T P iec e of the 1® *
-«•ignetization is i"
’ bQt °f molecnles- ■
2° «a***;
bem ®tW0,Sfl^
P*" "There©;
t 1 * *^§netiziiig iiua i
stianoli to the i
htse molecules®!'
md that the;
fe molecules so tiat
Q me direction, ii.
te modern theoiyi
nted out byti
molecnles do notti
of the magnet^
cted on by a Iorcei
, or because an ep
)n of the entirety!
mple expression i
en of in the last !
arising from the:
e with the delta
re D denotes a®
ecuiar directive t®
npère, the map®
irrent circulatiiigii:
rular currents is
rv to suppose that:
i the current,
1 of currents was ¡a
ned the phenoms-
d in the molecnle'
? " molecular mf'
Veber elements, ¡-
term suggests dm
lemical molecule. •
w„ voL 87 1 p- ^ "
III. 3]
is obviously not the case. In the older theories of paramagnetism
the Weber elements were supposed to turn under the influence of
the applied field so as to set their axes in the direction of the field,
and this tendency was supposed to be resisted by forces of a
quasi-elastic nature. Weber also introduced forces of a frictional
character to enable him to account for ferromagnetic effects.
It is not a little remarkable that in papers written in 1871
Wilhelm Weber pictured the hypothetical molecular current,
which Ampère had imagined to be continually flowing inside
molecules, as the rotation of light positive charges about heavy
negative ones. He wrote as follows : “ The relation of the
two particles as regards their motions is determined by the
ratio of their masses e and e', on the assumption that in e and
e' are included the masses of the ponderable atoms which are
attached to the electrical atoms. Let e be the positive electrical
particle ; let the negative be exactly equal and opposite, and
therefore denoted by — e (instead of e'). But let a ponderable
atom be attracted to the latter so that its mass is thereby so
greatly increased as to make the mass of the positive particle
vanishingly small in comparison. The particle — e may then
be thought of as at rest, and the particle + e as in motion about
the particle — e. The two unlike particles in the condition
described constitute, then, an Ampèrian molecular current.”
Weber’s model is identical with that assumed in the modern
electron theory save in the fact that it is now the negative
particle whose mass is negligible in comparison with that of
the positive, so that the roles of the particles are reversed.
Langevin has developed this electronic theory of magnetism in
which a negative electron circles in an orbit about a positively
charged nucleus.
But to return to Weber’s work on paramagnetism and ferro
magnetism. Sir Alfred Ewing showed that Weber’s assumption
of frictional forces was unnecessary. He constructed models
in which the controlling forces are magnetic only, and arise from
the action of the neighbouring molecular magnets. This theory,
put forward by Ewing in 1890, gave a qualitative explanation
of the principal phenomena of ferromagnetism. It explained
the three characteristic stages of magnetization as saturation is
approached (Fig. 4).
In the first stage the intensity of magnetization, I, increases
slowly and in proportion to the magnetizing field, H ; in the
second stage the curve showing the relation between I and H is
very steep, the value of I rising rapidly as H increases. After
this the increase in I is slow, and eventually as magnetic satura
tion is approached I becomes nearly constant.
When the Weber elements are caused to turn by an applied