8.
Now we come to the basic null-type or potentiometric meter (fig 5-4).
M1 is a galvanometer. Any time there is a difference of EMF between A and
B, current will flow through M1. The wiper arm of the voltage divider is
moved up or down, changing the applied null voltage until current does not
flow and the voltage at B is exactly equal to the voltage being measured.
Figure 5-4.
Basic null-type meter
9.
The voltage divider is much more complicated than shown, but the
results are the same.
The voltage output can be read directly to five
digits, using a mechanical readout system (example:
345.64 VDC).
As the
wiper arm is moved up or down, it mechanically changes these numbers to
correspond to the voltage of B.
10. Note in Figure 5-5 that when null is obtained (null being the point
where both voltages are the same and no current flows through the
galvanometer) that exactly the same load is felt by the reference supply,
regardless of where the voltage divider is set or what the input voltage is.
11. The only remaining point to cover is the measurement of AC voltage.
Shown in Figure 5-6 is a basic potentiometric meter with the following
important points: Will measure ac or dc; will not load circuit; because of
mechanical readout, resolution is extremely good; two people could never
read a 5-digit readout and obtain the same answer; and accuracy is better
than 0.1 percent on dc.
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