TESTING FOR CONDUCTIVITY AND

INSULATION.

Testing for Conductivity.-In making this test, the instruments are connected as represented in Fig, 136. B is the battery, G the galvanometer, and S is a coil of wire being tested for electrical continuity or conductivity. As will be seen, the

positive pole + of the battery is connected to one terminal of the galvanometer, and the other or negative pole is connected to one of the ends of the coil under test. The other terminal of the galvanometer is connected to the other end of the coil, If the connecting wires are making good electrical contact

TESTING FOR CONDUCTIVITY.

with the respective terminals, and the wire of coil being tested is unbroken, the needle of the galvanometer will be deflected as soon as a closed circuit is made by the end of the coil coming into contact with the galvanometer terminal. If the wire of the coil is broken in some part or the ends of the con

necting wires do not make good electrical contact with the terminals, the needle will not be deflected. In order to prevent mistakes, it is advisable to test the battery and galvanometer connections and contacts by short circuiting or bringing the ends of the wire connecting the terminal of the galvanometer and negative pole of the battery together before starting to test the circuit or coil. If the needle is deflected, the connections are all right; if undeflected, there is a bad contact some where, which must be made good before the test can proceed.

TESTING FOR CONDUCTIVITY.

Testing for Insulation.-The object of this test is to ascer tain whether the insulation of a circuit or of the wire wound upon a metal spool or core, such as a magnet core, has broken down or is in good order. In making the test, the instruments and connections are arranged as shown in Fig. 137. The battery and galvanometer are connected to one another, as in the conductivity test described above. The unconnected terminal of the battery is connected to one end of the coil under test, the other end of the coil remaining free and unconnected. Some portion of the metal core, say the end, is then cleaned bright with a knife or emery cloth, and the unconnected terminal of the galvanometer is brought into contact with this bright or clean part of the core. If then some portion of the insulation of the wire has been abraded or destroyed, thus bringing the bare wire into contact with the metal core, as at A in the figure, the needle of the galvanometer will be deflected since a closed circuit is formed through the core and wire. If on the contrary the insulation is perfect, the needle will be undeflected. It will thus be seen that in the conductivity test it is necessary that the needle should be deflected, or turned, to prove that all is right, while in the insulation test the converse holds good; if the needle is deflected, it proves that the insulation is broken down.

THE ALTERNATOR.

A dynamo constructed without a commutator for making the current direct, produces an alternating current, and such a machine is called an alternator. A great many kinds of alternators have been constructed. They may be thus classified:

I. Those with a stationary field magnet and rotating ar

mature.

2. Those with rotating field magnet and stationary armature.

3. Those with both field magnet and armature part stationary.

In the latter class, the amount of magnetic induction from the armature to the field is caused to vary or alternate in direction by the revolution of appropriate pieces of iron called inductors. Still another division rests on whether they give one simple alternating current, a two-phase current, or whether they give multiple currents.

In alternate-current working machines, the current is rapidly reversed, rising and falling in a succession of impulses or waves. Electricity is, in fact, oscillating backwards and