like an armature, rapidly and continuously through another field
of force, like a magnetic field.

DIRECT-CURRENT DYNAMO.--We shall now make the simplest form of dynamo,
using for this purpose a pair of permanent magnets.

[Illustration: _Fig. 22._ DYNAMO FIELD AND POLE PIECE]

SIMPLE MAGNET CONSTRUCTION.--A simple way to make a pair of magnets for
this purpose is shown in Fig. 22. A piece of round 3/4-inch steel core
(A), 5-1/2 inches long, is threaded at both ends to receive at one end a
nut (B), which is screwed on a sufficient distance so that the end of
the core (A) projects a half inch beyond the nut. The other end of the
steel core has a pole piece of iron (C) 2" x 2" x 4", with a hole
midway between the ends, threaded entirely through, and provided along
one side with a concave channel, within which the armature is to turn.
Now, before the pole piece (C) is put on, we will slip on a disc (E),
made of hard rubber, then a thin rubber tube (F), and finally a rubber
disc (G), so as to provide a positive insulation for the wire coil which
is wound on the bobbin thus made.

HOW TO WIND.--In practice, and as you go further along in this work, you
will learn the value, first, of winding one layer of insulated wire on
the spool, coating it with shellac, and then putting on the next layer,
and so on; when completely wound, the two wire terminals may be brought
out at one end; but for our present purpose, and to render the
explanation clearer, the wire terminals are at the opposite ends of the
spool (H, H').

THE DYNAMO FIELDS.--Two of these spools are so made and they are called
the _fields_ of the dynamo.

We will next prepare an iron bar (I), 5 inches long and 1/2 inch thick
and 1-1/2 inches wide, then bore two holes through it so the distance
measures 3 inches from center to center. These holes are to be threaded
for the 3/4-inch cores (A). This bar holds together the upper ends of
the cores, as shown in Fig. 23.

[Illustration: _Fig. 23._ BASE AND FIELDS ASSEMBLED]

We then prepare a base (J) of any hard wood, 2 inches thick, 8 inches
long and 8 inches wide, and bore two 3/4-inch holes 3 inches apart on a
middle line, to receive a pair of 3/4-inch cap screws (K), which pass
upwardly through the holes in the base and screw into the pole pieces
(C). A wooden bar (L), 1-1/2" x 1-1/2", 8 inches long, is placed under
each pole piece, which is also provided with holes for the cap screws
(K). The lower side of the b