a given time, and the current induced in the
coil is therefore now most intense. Here we must stop a moment to
consider how to decide in which direction the current flows. The
armature is revolving in a clockwise direction, and _y z_, therefore, is
moving downwards. Now, suppose that you rest your _left_ hand on the N.
pole of the magnet so that the arm lies in a line with the magnet. Point
your forefinger towards the S. pole. It will indicate the _direction of
the lines of force_. Bend your other three fingers downwards over the
edge of the N. pole. They will indicate the _direction in which the
conductor is moving_ across the magnetic field. Stick out the thumb at
right angles to the forefinger. It points in the direction in which the
_induced_ current is moving through the nearer half of the coil.
Therefore lines of force, conductor, and induced current travel in
planes which, like the top and two adjacent sides of a box, are at right
angles to one another.

While current travels from _z_ to _y_--that is, _from_ the ring C^1 to
_y_--it also travels from _x_ to _w_, because _w x_ rises while _y z_
descends. So that a current circulates through the coil and the exterior
part of the circuit, including the lamp. After _z y_ has passed the
lowest possible point of the circle it begins to ascend, _w x_ to
descend. The direction of the current is therefore reversed; and as the
change is repeated every half-revolution this form of dynamo is called
an _alternator_ or creator of alternating currents. A well-known type of
alternator is the magneto machine which sends shocks through any one who
completes the external circuit by holding the brass handles connected by
wires to the brushes. The faster the handle of the machine is turned the
more frequent is the alternation, and the stronger the current.

[Illustration: FIG. 69.]

CONTINUOUS-CURRENT DYNAMOS.

An alternating current is not so convenient for some purposes as a
continuous current. It is therefore sometimes desirable (even necessary)
to convert the alternating into a uni-directional or continuous current.
How this is done is shown in Figs. 69 and 70. In place of the two
collecting rings C C^1, we now have a single ring split longitudinally
into two portions, one of which is connected to each end of the coil _w
x y z_. In Fig. 69 brush B has just passed the gap on to segment C,
brush B^1 on to segment C^1. For half a revolution these remain
respectively in contact