SUMMARY

The important points of this chapter are summarized in the following paragraphs. You should be familiar with these points before continuing with your study of test equipment.

A permanent-magnet, moving-coil meter movement (D'ARSONVAL movement) uses the interaction of magnetic fields to produce movement.

DAMPING is used to smooth out the vibration and to help prevent overshooting of the meter pointer.

ELECTRODYNAMOMETER movements are usually used in wattmeters. They operate much like the D'Arsonval meter movement, except field coils are used instead of a permanent magnet. Electrodynamometer movements measure either ac or dc without the use of a rectifier.

A SHUNT is a physically large, low-resistance conductor connected in parallel with the meter terminals. It carries the majority of the load current so that only a small portion of the total current will flow through the meter coil.

An AMMETER measures current and is always connected in series with the circuit being measured. An ammeter should have a low resistance so that the effect of the ammeter on the circuit will be kept to a minimum.

VOLTMETERS are used to measure voltage and are always connected in parallel with the circuit being measured. A voltmeter should have a high resistance compared to the circuit being measured to minimize the loading effect. Voltmeter sensitivity is expressed in ohms per volt.

OHMMETERS are used to measure resistance and to check continuity. An ohmmeter is electrically connected in series with the resistance being measured. The ohmmeter range, which allows a midscale deflection, should be used.

A MEGOHMMETER (MEGGER) is used to measure very high resistance, such as the insulation of wiring.

A WATTMETER is usually an electrodynamometer and is used to measure power.

A CONTINUITY TEST is accomplished with an ohmmeter. This test is used to check for opens (or to see if the circuit is complete or continuous).

GROUNDED CIRCUITS are caused by some conducting part of the circuit making contact either directly or indirectly with the metallic structure of the ship or chassis. In testing for grounds, you may use either an ohmmeter or a megger.

A SHORT CIRCUIT, other than a grounded one, is where two conductors touch each other directly or through another conducting element. An ohmmeter is used to test for shorts.