; then, just as _y z_ begins to rise and _w x_ to
descend, the brushes cross the gaps again and exchange segments, so that
the current is perpetually flowing one way through the circuit. The
effect of the commutator[17] is, in fact, equivalent to transposing the
brushes of the collecting rings of the alternator every time the coil
reaches a zero position.

Figs. 71 and 72 give end views in section of the coil and the
commutator, with the coil in the position of minimum and maximum
efficiency. The arrow denotes the direction of movement; the double
dotted lines the commutator end of the revolving coil.

[Illustration: FIG. 70.]

PRACTICAL CONTINUOUS-CURRENT DYNAMOS.

The electrical output of our simple dynamo would be increased if,
instead of a single turn of wire, we used a coil of many turns. A
further improvement would result from mounting on the shaft, inside the
coil, a core or drum of iron, to entice the lines of force within reach
of the revolving coil. It is evident that any lines which pass through
the air outside the circle described by the coil cannot be cut, and are
wasted.

[Illustration: FIG. 71.]

[Illustration: FIG. 72.]

The core is not a solid mass of iron, but built up of a number of very
thin iron discs threaded on the shaft and insulated from one another to
prevent electric eddies, which would interfere with the induced current
in the conductor.[18] Sometimes there are openings through the core from
end to end to ventilate and cool it.

[Illustration: FIG. 73.]

We have already noticed that in the case of a single coil the current
rises and falls in a series of pulsations. Such a form of armature would
be unsuitable for large dynamos, which accordingly have a number of
coils wound over their drums, at equal distances round the
circumference, and a commutator divided into an equal number of
segments. The subject of drum winding is too complicated for brief
treatment, and we must therefore be content with noticing that the coils
are so connected to their respective commutator segments and to one
another that they mutually assist one another. A glance at Fig. 73 will
help to explain this. Here we have in section a number of conductors on
the right of the drum (marked with a cross to show that current is
moving, as it were, into the page), connected with conductors on the
left (marked with a dot to signify current coming out of the page). If
the "crossed" and "dotted" conductors were respec