452

ELECTRICAL APPARATUS

induction is in D, and C is the return, but the voltage in the cir-
cuit, CD, is the same. If, then, C and D both stand still, either
there is no induction in either, or, assuming the lines of magnetic
force to revolve, equal and opposite voltages are induced in C
and D, and the voltage in circuit, CD, is zero just the same.
However, the question whether the lines of force of a revolving
magnet rotate or not, is meaningless for this reason: the lines of
force are a pictorial representation of the magnetic field in space.
The magnetic field at any point is characterized by an intensity
and a-direction, and as long as intensity and direction at any point
are constant or stationary, the magnetic field is constant or sta-
tionary. This is the case in Figs. 215 and 216, regardless whether
the magnet revolves around its axis or not, and the rotation of
the magnet thus has no effect whatsoever on the induction phe-
nomena. The magnetic field is stationary at any point of space
outside of the magnet, and it is also stationary at any point of
space inside of the magnet, even if the magnet revolves, and at
the same time it is stationary also with regards to any element
of the revolving magnet. Using then the pictorial representation
of the lines of magnetic force, we can assume these lines of force
as stationary in space, or as revolving with the rotating magnet,
whatever best suits the convenience of the problem at hand: but
whichever assumption we make, makes no difference on the solu-
tion of the problem, if we reason correctly from the assumption.
248. As in the unipolar machine each conductor (correspond-
ing to a half turn of the bipolar or multipolar machine) requires
a separate high-speed collector ring, many attempts have
been made (and are still being made) to design a coil-wound
unipolar machine, that is, a machine connecting a number of
peripheral conductors in series, without going through collector
rings. This is an impossibility, and unipolar induction, that in,
continues induction of a unidirectional voltage, is possible only
in an open conductor, but not in a coil or turn, as the voltage
electro magnetically induced in a coil or turn must always be an
alternating voltage.
The fundamental law of electromagnetic induction is, that the
induced voltage is proportional to the rate of cutting of the con-
ductor through the lines of force of the magnetic field. Applying
this to a clc-sed circuit or turn: every line of magnetic force cut
by a turn must either go from the outside to the inside, or from
the inside to the outside of the turn. This means: the voltage