They may be compared to the molecules of a gas which is
enclosed in a porous body. In ordinary conditions, notwithstanding the
great speed with which they are animated, they are unable to travel
long distances, because they quickly find their road barred by a
material atom. They have to undergo innumerable impacts, which throw
them first in one direction and then in another. The passage of a
current is a sort of flow of these electrons in a determined
direction. This electric flow brings, however, no modification to the
material medium traversed, since every electron which disappears at
any point is replaced by another which appears at once, and in all
metals the electrons are identical.
This hypothesis leads us to anticipate certain facts which experience
confirms. Thus J.J. Thomson shows that if, in certain conditions, a
conductor is placed in a magnetic field, the ions have to describe an
epicycloid, and their journey is thus lengthened, while the electric
resistance must increase. If the field is in the direction of the
displacement, they describe helices round the lines of force and the
resistance is again augmented, but in different proportions.
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