Figure 1-12.
Determining value of unknown resistance of voltmeter
When power is applied, the voltage (Em) across the meter can be read on the
voltmeter.
Then by shorting out resistor Rx (fig 1-11), the voltmeter indicates
the applied voltage (Eb).
Thus the voltage (Ex) across the unknown resistor is
equal to the difference between the two measured voltages.
Ex = Eb=-Em
c. The voltages across the resistor and the meter are
Ex = IRx and Em=IRm
d. By dividing the first equation by the second equation, the currents cancel
out:
e. But since Ex is equal to the difference between Em and Eb (Eb-Em) the
unknown resistance is
f. Since Eb, Em, and Rm are unknown, the above equation can be solved for the
value of the resistance Rx.
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