A moving coil galvanometer gives full scale deflection, when...

A moving coil galvanometer gives full scale deflection, when a current of $0.005 \mathrm{~A}$ is passed through its coil. It is converted into a voltmeter reading upto $5 \mathrm{~V}$ by using an external resistance of $975 \Omega$. What is the resistance of the galvanometer coil? (a) $30 \Omega$ (b) $25 \Omega$ (c) $50 \Omega$ (d) $40 \Omega$

A moving coil galvanometer gives full scale deflection, when a current of $0.005 \mathrm{~A}$ is passed through its coil. It is converted into a voltmeter reading upto $5 \mathrm{~V}$ by using an external resistance of $975 \Omega$. What is the resistance of the galvanometer coil?
(a) $30 \Omega$
(b) $25 \Omega$
(c) $50 \Omega$
(d) $40 \Omega$

Key Concepts

Ohm's Law

Ohm's Law, which states that voltage equals the product of current and resistance (V = IR), is fundamental in analyzing circuits like the modified galvanometer setup. It is used to relate the applied voltage, current through the device, and the combined resistance of the galvanometer and the series resistor, allowing one to determine unknown resistance values in the circuit.

Moving Coil Galvanometer

A moving coil galvanometer is an instrument that measures small electric currents by deflecting a needle about a pivot. Its sensitivity is based on the interaction between the current passing through a coil and an external magnetic field, making it a precise tool for detecting and measuring low currents.

Full-scale Deflection

Full-scale deflection refers to the maximum reading or maximum deflection of the galvanometer when the current reaches a predetermined value. This specification is crucial because it defines the sensitivity and limits within which the device operates reliably, and it serves as a calibration point when converting the instrument to measure different electrical quantities.

Conversion into a Voltmeter Using a Series Resistor

To convert a galvanometer into a voltmeter, a high resistance is added in series with the galvanometer. This combination allows the device to measure higher voltages by ensuring that only a fraction of the current passes through the sensitive galvanometer element, while the extra resistance drops the remaining voltage. The value of this series resistor, in conjunction with the known full-scale deflection current, is calculated to extend the measurement range properly.

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