, depends upon the speed of rotation of the loop
past the end of the magnet.

[Illustration: _Figs. 107-108._ FORM FOR INCREASING ALTERNATIONS]

Instead, therefore, of using a single loop, we may make four loops (Fig.
107), which at the same speed as we had in the case of the single loop,
will give four alternations, instead of one, and still further, to
increase the periods of alternation, we may use the four loops and two
magnets, as in Fig. 108. By having a sufficient number of loops and of
magnets, there may be 40, 50, 60, 80, 100 or 120 such alternating
periods in each second. Time, therefore, is an element in the operation
of alternating currents.

Let us now illustrate the manner of connecting up and building the
dynamo, so as to derive the current from it. In Fig. 109, the loop (A)
shows, for convenience, a pair of bearings (B). A contact finger (C)
rests on each, and to these the circuit wire (D) is attached. Do not
confuse these contact fingers with the commutator brushes, shown in the
direct-current motor, as they are there merely for the purpose of making
contact between the revolving loop (A) and stationary wire (D).

[Illustration: _Fig. 109._ CONNECTION OF ALTERNATING DYNAMO ARMATURE]

BRUSHES IN A DIRECT-CURRENT DYNAMO.--The object of the brushes in the
direct-current dynamo, in connection with a commutator, is to convert
this _inductance_ of the wire, or this effort to reverse itself into a
current which will go in one direction all the time, and not in both
directions alternately.

To explain this more fully attention is directed to Figs. 110 and 111.
Let A represent the armature, with a pair of grooves (B) for the wires.
The commutator is made of a split tube, the parts so divided being
insulated from each other, and in Fig. 110, the upper one, we shall call
and designate the positive (+) and the lower one the negative (-). The
armature wire (C) has one end attached to the positive commutator
terminal and the other end of this wire is attached to the negative
terminal.

[Illustration: _Fig. 110._ DIRECT CURRENT DYNAMO]

One brush (D) contacts with the positive terminal of the commutator and
the other brush (E) with the negative terminal. Let us assume that the
current impulse imparted to the wire (C) is in the direction of the dart
(F, Fig. 110). The current will then flow through the positive (+)
terminal of the commutator to the brush (D), and from the brush (D)
through the wire (G) to the