circuit E, and its resistance therefore higher, the current in the
shunt circuit M is greater than before, the magnets become
stronger, and the electromotive force of the armature is
increased. The Edison machine is of this type, and is illustrated
in figure 43, where M M' are the field magnets with their poles N
S, between which the armature A is revolved by means of the belt
B, and a pulley seen behind. The leading wires W W convey the
current from the brushes of the commutator to the external
circuit. In this machine the conductors of the armature are not
coils of wire, but separate bars of copper.

In shunt machines the variation of current due to a varying number
of lamps in use occasions a rise and fall in the brightness of the
lamps which is undesirable, and hence a third class of dynamo has
been devised, which combines the principles of both the series and
shunt machines. This is the "compound-wound" machine, in which the
magnets are wound partly in shunt and partly in series with the
armature, in such a manner that the strength of the field-magnets
and the electromotive force of the current do not vary much,
whatever be the number of lamps in circuit. In alternate current
machines the electromotive force keeps constant, as the field-
magnets are excited by a separate machine, giving a continuous
current.

We have already seen that the action of the dynamo is reversible,
and that just as a wire moved across a magnetic field supplies an
electric current, so a wire at rest, but conducting a current
across a magnetic field, will move. The electric motor is
therefore essentially a dynamo, which on being traversed by an
electric current from an external source puts itself in motion.