brush (E), which contacts with the negative
(-) terminal of the commutator. This will continue to be the case, while
the wire (C) is passing the magnetic field, and while the brush (D) is
in contact with the positive (+) terminal. But when the armature makes a
half turn, or when it reaches that point where the brush (D) contacts
with the negative (-) terminal, and the brush (E) contacts with the
positive (+) terminal, a change in the direction of the current through
the wire (G) takes place, unless something has happened to change it
before it has reached the brushes (D, E).

[Illustration: _Fig. 111._ CIRCUIT WIRES IN DIRECT CURRENT DYNAMO]

Now, this change is just exactly what has happened in the wire (C), as
we have explained. The current attempts to reverse itself and start out
on business of its own, so to speak, with the result that when the
brushes (D and E) contact with the negative and positive terminals,
respectively, the surging current in the wire (C) is going in the
direction of the dart (H)--that is, while, in Fig. 110, the current
flows from the wire (C) into the positive terminal, and out of the
negative terminal into the wire (C), the conditions are exactly reversed
in Fig. 111. Here the current in wire C flows _into_ the negative (-)
terminal, and _from_ the positive (+) terminal into the wire C, so that
in either case the current will flow out of the brush D and into the
brush E, through the external circuit (G).

It will be seen, therefore, that in the direct-current motor, advantage
is taken of the surging, or back-and-forth movement, of the current to
pass it along in one direction, whereas in the alternating current no
such change in direction is attempted.

ALTERNATING POSITIVE AND NEGATIVE POLES.--The alternating current,
owing to this surging movement, makes the poles alternately positive and
negative. To express this more clearly, supposing we take a line (A,
Fig. 112), which is called the zero line, or line of no electricity. The
current may be represented by the zigzag line (B). The lines (B) above
zero (A) may be designated as positive, and those below the line as
negative. The polarity reverses at the line A, goes up to D, which is
the maximum intensity or voltage above zero, and, when the current falls
and crosses the line A, it goes in the opposite direction to E, which is
its maximum voltage in the other direction. In point of time, if it
takes one second for the current to go from