Which instrument is used to detect the presence of an electric current in a circuit?

A galvanometer is a sensitive instrument used to detect the presence of a small electric current in a circuit and to show its direction. It is named after the Italian scientist Luigi Galvani, who studied "animal electricity" in frogs' legs in the 1780s. The first galvanometer was made by Johann Schweigger in 1820, soon after Hans Christian Ørsted discovered that an electric current produces a magnetic field.

The most common type is the moving coil galvanometer. It consists of a rectangular coil of fine copper wire suspended or pivoted between the poles of a strong permanent magnet. When a current flows through the coil, the magnetic field exerts a torque on it, making it rotate. A pointer attached to the coil moves over a scale. The deflection is proportional to the current. A spring brings the pointer back to zero when the current stops. In many galvanometers, zero is at the centre of the scale, so the pointer can move to the left or right, showing the direction of the current.

A galvanometer is very sensitive and can be damaged by large currents, so it is used mainly to detect current, not to measure large values. However, it can be converted into other instruments:

Into an ammeter, by connecting a low resistance (shunt) in parallel with it.

Into a voltmeter, by connecting a high resistance in series with it.

Galvanometers are used in experiments such as electromagnetic induction. In Faraday's experiments, a galvanometer connected to a coil shows a deflection when a magnet is moved into or out of the coil, proving that a changing magnetic field produces a current.

Why not the other options?

• ✗A voltmeter measures potential difference.
• ✗A barometer measures atmospheric pressure.
• ✗An ohmmeter measures resistance.