wo parts together. Near the upper end of
the pin (L) solder the end of a pointer (O), this pointer being at right
angles to the armature (K). It is better to have a metal socket for the
lower end of the pin. When these parts are put together, as shown in
Fig. 37, a removable glass top, or cover, should be provided.

This is shown in Fig. 40, in which a square, wooden frame (P) is used,
and a glass (Q) fitted into the frame, the glass being so arranged that
when the cover is in position it will be in close proximity to the upper
projecting end of the pivot pin (L), and thus prevent the magnet from
becoming misplaced.

[Illustration: _Fig. 40._ CROSS SECTION OF DETECTOR]

HOW TO PLACE THE DETECTOR.--If the detector is placed north and south,
as shown by the two markings, N and S (Fig. 37), the magnet bar will
point north and south, being affected by the earth's magnetism; but when
a current of electricity flows through the coil (B), the magnet will be
deflected to the right or to the left, so that the pointer (O) will then
show the direction in which the current is flowing through the wire (R)
which you are testing.

The next step of importance is to _measure_ the current, that is, to
determine its strength or intensity, as well as the flow or quantity.

DIFFERENT WAYS OF MEASURING A CURRENT.--There are several ways to
measure the properties of a current, which may be defined as follows:

1. THE SULPHURIC ACID VOLTAMETER.--By means of an electrolytic action,
whereby the current decomposes an acidulated solution--that is, water
which has in it a small amount of sulphuric acid--and then measuring the
gas generated by the current.

2. THE COPPER VOLTAMETER.--By electro-chemical means, in which the
current passes through plates immersed in a solution of copper sulphate.

3. THE GALVANOSCOPE.--By having a coil of insulated wire, with a magnet
suspended so as to turn freely within the coil, forming what is called a
galvanoscope.

4. ELECTRO-MAGNETIC METHOD.--By using a pair of magnets and sending a
current through the coils, and then measuring the pull on the armature.

5. THE POWER OR SPEED METHOD.--By using an electric fan, and noting the
revolutions produced by the current.

6. THE CALORIMETER.--By using a coil of bare wire, immersed in paraffine
oil, and then measuring the temperature by means of a thermometer.

[Illustration: _Fig. 41._ ACID VOLTAMETER]

[Illustration: _Fig. 42._ COPPER VOLTAMETER]

7. THE