(2) Limitations which prevent a further decrease in the range are:
(a) Internal resistance of the battery which becomes an appreciable part
of the total circuit resistance, causing errors in readings to increase with
battery age.
(b) Excessive current drain which decreases the life of the battery, and
which could in some cases cause damage to the components under test.
(3) It is impractical to measure very low values of resistance with an
ohmmeter.
The ohmmeter, in itself, is only a means of approximating resistance
values, where practical electronic work requires a convenient and speedy method of
checking resistances.
Learning Event 2:
DETERMINE WHEN AND HOW TO USE METERS
1.
enough to keep the deflection less than full scale. Before measuring a current or
voltage, you should have some idea of its magnitude. Then switch to a large enough
range or start with the highest range.
NOTE:
Many voltmeters or ohmmeters have been ruined by
attempting to measure amperes. Therefore, be sure to read
the lettering either on the dial or on the switch
positions, and choose the proper range before connecting
the ammeter in the circuit.
2.
When connecting the voltmeter (or ammeter) in the circuit, you must observe
proper polarity.
Current must flow through the coil in a definite direction in
order to move the indicator needle up-scale.
Wrong polarity (or reversal of
current) results in a bent meter needle. You can avoid improper meter connections
by remembering that the black meter leads are the negative leads and the red meter
leads are the positive leads.
3.
There are other precautions to observe with both the ammeter and
the
voltmeter. When using an ammeter, you must observe two important precautions.
a. Always connect an ammeter in series with the element through which the
current is to be measured.
b. Never connect an ammeter across a source of voltage, such as a battery or
a generator.
c. Remember that the resistance of an ammeter, particularly on the higher
ranges, is extremely low and that any voltage, even a volt or less, may cause very
high current through the meter, and severely damage the meter.
4.
When using a voltmeter you must observe one additional precaution along with
those already mentioned. Always connect a voltmeter in parallel across
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