tively the "up" and
"down" turns of a single coil terminating in a simple split commutator
(Fig. 69), when the coil had been revolved through an angle of 90 deg.
some of the up turns would be ascending and some descending, so that
conflicting currents would arise. Yet we want to utilize the whole
surface of the drum; and by winding a number of coils in the manner
hinted at, each coil, as it passes the zero point, top or bottom, at
once generates a current in the desired direction and reinforces that in
all the other turns of its own and of other coils on the same side of a
line drawn vertically through the centre. There is thus practically no
fluctuation in the pressure of the current generated.

The action of single and multiple coil windings may be compared to that
of single and multiple pumps. Water is ejected by a single pump in
gulps; whereas the flow from a pipe fed by several pumps arranged to
deliver consecutively is much more constant.

MULTIPOLAR DYNAMOS.

Hitherto we have considered the magnetic field produced by one bi-polar
magnet only. Large dynamos have four, six, eight, or more field magnets
set inside a casing, from which their cores project towards the armature
so as almost to touch it (Fig. 74). The magnet coils are wound to give
N. and S. poles alternately at their armature ends round the field; and
the lines of force from each N. pole stream each way to the two adjacent
S. poles across the path of the armature coils. In dynamos of this kind
several pairs of collecting brushes pick current off the commutator at
equidistant points on its circumference.

[Illustration: FIG. 74.--A Holmes continuous current dynamo: A,
armature; C, commutator; M, field magnets.]

EXCITING THE FIELD MAGNETS.

Until current passes through the field magnet coils, no magnetic field
can be created. How are the coils supplied with current? A dynamo,
starting for the first time, is excited by a current from an outside
source; but when it has once begun to generate current it feeds its
magnets itself, and ever afterwards will be self-exciting,[19] owing to
the residual magnetism left in the magnet cores.

[Illustration: FIG. 75.--Partly finished commutator.]

Look carefully at Figs. 77 and 78. In the first of these you will
observe that part of the wire forming the external circuit is wound
round the arms of the field magnet. This is called a _series_ winding.
In this case _all_ the current generated helps to excite