is continuous, or constant in its direction.

The current of the series machine varies with the resistance of
the external or working circuit, because that is included in the
circuit of the field magnets and the armature. Thus, if we vary
the number of electric lamps fed by the machine, we shall vary the
current it is capable of yielding. With arc lamps in series, by
adding to the number in circuit we increase the resistance of the
outer circuit, and therefore diminish the strength of the current
yielded by the machine, because the current, weakened by the
increase of resistance, fails to excite the field magnets as
strongly as before. On the other hand, with glow lamps arranged in
parallel, the reverse is the case, and putting more lamps in
circuit increases the power of the machine, by diminishing the
resistance of the outer circuit in providing more cross-cuts for
the current. This, of course, is a drawback to the series machine
in places where the number of lamps to be lighted varies from time
to time. In the "shunt-wound" machine the field magnets are
excited by diverting a small portion of the main current from the
armature through them, by means of a "shunt" or loop circuit. Thus
in figure 42 where C is the commutator and b b' the brushes, M is
a shunt circuit through the magnets, and E is the external or
working circuit of the machine.

The small arrows indicate the directions of the currents. With
this arrangement the addition of more glow lamps to the external
circuit E DIMINISHES the current, because the portion of it which
flows through the by-path M, and excites the magnets, is less now
that the alternative route for the current through E is of lower
resistance than before. When fewer glow lamps are in the external