the work required.
This is a more useful form of winding for electroplating purposes.

[Illustration: _Fig. 32._ SHUNT-WOUND _Fig. 32._ COMPOUND-WOUND]

COMPOUND-WOUND FIELD.--Fig. 33 is a diagram of a "compound-wound"
dynamo. The regular field winding (J) has its opposite ends connected
directly with the armature brushes. There is also a winding, of a
comparatively few turns, of a thicker wire, one terminal (K) of which is
connected with one of the brushes and the other terminal (K') forms one
side of the lighting circuit. A wire (L) connects with the other
armature brush to form a complete lighting circuit.

CHAPTER V

HOW TO DETECT AND MEASURE ELECTRICITY

MEASURING INSTRUMENTS.--The production of an electric current would not
be of much value unless we had some way by which we might detect and
measure it. The pound weight, the foot rule and the quart measure are
very simple devices, but without them very little business could be
done. There must be a standard of measurement in electricity as well as
in dealing with iron or vegetables or fabrics.

As electricity cannot be seen by the human eye, some mechanism must be
made which will reveal its movements.

THE DETECTOR.--It has been shown in the preceding chapter that a current
of electricity passing through a wire will cause a current to pass
through a parallel wire, if the two wires are placed close together, but
not actually in contact with each other. An instrument which reveals
this condition is called a _galvanometer_. It not only detects the
presence of a current, but it shows the direction of its flow. We shall
now see how this is done.

For example, the wire (A, Fig. 35) is connected up in an electric
circuit with a permanent magnet (B) suspended by a fine wire (C), so
that the magnet (B) may freely revolve.

[Illustration: _Fig. 34._ _Fig. 35._ _Fig. 36._
TO THE RIGHT, COMPASS MAGNET, TO THE LEFT]

For convenience, the magnetic field is shown flowing in the direction of
the darts, in which the dart (D) represents the current within the
magnet (B) flowing toward the north pole, and the darts (E) showing the
exterior current flowing toward the south pole. Now, if the wire (A) is
brought up close to the magnet (B), and a current passed through A, the
magnet (B) will be affected. Fig. 35 shows the normal condition of the
magnetized bar (B) parallel with the wire (A) when a current is not
passing through the latter.