Figure 5-1.
Passive meter loading
Note that when this meter is placed across the resistor, it actually creates
an alternate 10-kilohm current path around R2.
Where the total circuit
resistance was 20,000 ohms with a current flow of 0.5 milliampere, now there
is a total resistance of 15,000 ohms and the current flow is 0.67
milliampere. (Ten volts divided by 20 kilohms equals 0.5 milliampere and 10
volts divided by 15 kilohms equals 0.6667 milliampere.) This is what is
meant by loading. Where the voltage dropped across R2 was 5 volts and it
could have been in tolerance, the meter will indicate close to 3.3 volts, or
out of tolerance by 35 percent.
4.
A way of greatly reducing this loading problem is to use a very high
impedance input circuit to the meter.
However, it should be pointed out
that there is still some loading, although very minor. By using a Vacuum
Tube Volt Meter (VTVM), you do reduce the effects of loading; however,
because of the extreme cost involved, most VTVMs are about 3 percent. So we
have lost our accuracy. What we need then is a meter with an accuracy of
better than 0.1 percent and one that produces no loading effect.
5.
Suppose we were to connect a battery of exactly the same potential as
that dropped across R1 as shown in Figure 5-2. No current would flow from A
to B, but we would be able to measure the actual voltage present at A with
no loading of the original circuit.
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