Ammeters are commonly incorporated in multiple purpose instruments such as multimeter or volt-ohm-milliammeters. These instruments vary somewhat according to the design used by different manufacturers, but most incorporate the functions of an ammeter, a voltmeter, and an ohmmeter in one unit. A typical multimeter is shown in figure 8-131.
This multimeter has two selector switches: a function switch and a range switch. Since a multimeter is actually three meters in one case, the function switch must be placed in proper position for the type of measurement to be made. In figure 8-131, the function switch is shown in the ammeter position to measure dc milliamperes and the range switch is set at 1000. Set in this manner, the ammeter can measure up to 1,000 milliamperes or 1 ampere.
|Multimeters have several scales, and the one used should correspond
properly to the position of the range switch. If current of unknown value
is to be measured, always select the highest possible range to avoid damage
to the meter. The test leads should always be connected to the meter in
the manner prescribed by the manufacturer. Usually the red lead is positive
and the black lead is negative, or common. Many multimeters employ color
coded jacks as an aid in connecting the meter into the circuit to be tested.
In figure 8-132, a multimeter properly set to measure current flow is connected
into a circuit.
The precautions to be observed when using an ammeter are summarized as follows:
1. Always connect an ammeter in series with the element through which the current flow is to be measured.
2. Never connect an ammeter across a source of voltage, such as a battery or generator. Remember that the resistance of an ammeter, particularly on the higher ranges, is extremely low and that any voltage, even a volt or so, can cause very high current to flow through the meter, causing damage to it.
3. Use a range large enough to keep the deflection less than full scale. Before measuring a current, form some idea of its magnitude. Then switch to a large enough scale or start with the highest range and work down until the appropriate scale is reached. The most accurate readings are obtained at approximately half scale deflection. Many milliammeters have been ruined by attempts to measure amperes. Therefore, be sure to read the lettering either on the dial or on the switch positions and choose proper scale before connecting the meter in the circuit.
4. Observe proper polarity in connecting the meter in the circuit. Current must flow through the coil in a definite direction in order to move the indicator needle up scale. Current reversal because of incorrect connection in the circuit results in a reversed meter deflection and frequently causes bending of the meter needle. Avoid improper meter connections by observing the polarity markings on the meter.