Galvanometer:
A galvanometer is an electrical device that detects and measures a small amount of current. It works on the principle of torque produced on a rectangular current-carrying coil placed inside the magnetic field.
Conversion of galvanometer into voltmeter:
The galvanometer is converted into the voltmeter by connecting a high resistance R in a series with the galvanometer. The voltage across the voltmeter is
V = Ig (G + R)
Or, \(R=\frac{R}{Ig}-G\)
Knowing the voltage of V,\(I_g\), and G, the value of R can be calculated.
The resistance of a voltmeter should be high
The voltmeter is a high resistance electrical device that is connected in parallel in a circuit and measures the potential difference across any load. The voltmeter has very high resistance because it is connected in parallel across any component of a circuit; it draws a very small current from the main circuit and most of the current pass through the element. Hence, the potential difference across the component is not affected. If the resistance is small, the effective resistance is affected and most of the current passed through the voltmeter and the potential difference reduces across the component and the voltmeter cannot measure accurately.