A D'Arsonval meter with a large-valued resistor put in series with it can be used to measure voltage. Draw a circuit representation of that and explain why it can be done so.

Question

kropot72 · Answer

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In the drawing V is the voltage to be measured, R is the large-valued series resistor and M is the meter.
Current drawn from the source of voltage flows through the resistor and meter and causes the meter to deflect.
If it assumed that 50 mico-amps will cause full-scale deflection of the meter, and 10 volts will also cause full-scale deflection of the meter, the total series resistance must be
$$R=\frac{V}{I}=\frac{10}{50\times 10^{-6}}=200,000ohms$$
The scale of the meter can be calibrated in voltage rather than current. 
It must be considered when measuring voltage with this arrangement that the voltage to be measured will have a resistance of 200,000 ohms in parallel. This loading will sometimes give a voltage reading that is a little lower than the voltage present without the resistor and meter connected.