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 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.