Galvanometer: A Galvanometer is an electrical device which detects the electrical current in a circuit.It also gives the direction of current in a circuit and can measure small amount of current. When a current is passed through the coil of a galvanometer, it experience torque and is deflected through an angle from its mean position. The deflection depends upon the amount of current passed through it. Galvanometer is denoted by symbol G.
Shunt: A shunt is a small resistance connected to a connected in parallel to galvanometer. It is generally used for increasing the range of galvanometer and current in the circuit.
Ammeter: An ammeter is a measuring instrument used to measure the current in a circuit. It also uses the magnetic deflection principle. It is denoted by A. Ammeter is replaced by multimeter these days.
Voltmeter: A voltmeter is an instrument used for measuring electrical potential difference between two points in an electric circuit. Voltmeter is represented by V. It measure the potential difference of both a.c and d.c. circuit. The internal resistance of a voltmeter is very high. It’s outer structure is same as that of ammeter and galvanometer.