Figure 5-2.
Basic null
6.
How do we know when the opposing voltage (second battery) or the
nulling voltage is exactly the same as that across R1? Note the hookup in
Figure 5-3.
If any difference in EMF exists between points A and B, a
current will flow in one direction through galvanometer M2. Thus by varying
R4 until M2 indicates no current flow, we can set the potential at A to
equal that at B. Then we will measure EMF at B with M1 and, in effect, we
have measured the EMF at point A with no loading of the original circuit.
Figure 5-3.
Null with galvanometer
7.
However, we have two problems. Varying R4 presents a varying load to
B2, which would present a problem if B2 were replaced by a power supply. M1
cannot give us any greater accuracy than the meter itself, or about 3
percent.
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