The differential voltmeter technique is one among the foremost common and accurate method of measuring unknown voltage. During this technique, the voltmeter is employed to point the difference between known and unknown voltages i.e. an unknown voltage is compared to a known voltage.
The basic sort of differential voltmeter is shown within the figure.
In this method, the potentiometer is adjusted to get zero within the null indicator. Under this balance condition. The unknown voltage is adequate to the voltage output of the potentiometer under null conditions, the meter draws current form neither the reference source nor the unknown voltage source, and hence the differential voltmeter present an infinite impedance to the unknown source. The potentiometer features a calibrated front panel. The null detector is calibrated in difference of the voltages. Just in case prefect balance can't be obtained the unknown voltage is given by the sum of the potentiometer and null detector readings.
The reference
source used is typically a 1V dc standard source (or) zener controlled
precision supply. A high voltage reference supply is employed for measuring
high voltages. Alternatively, a potential divider (or) attenuator is usually
wont to attenuate the voltage for direct comparison against. the standard low
voltage dc standard. But, the most drawback of this method is that the low
input resistance of the voltmeter, especially for higher voltage which is
undesirable due to its loading effect.
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