CBSE Class 12-science Answered

Dear student,

Galvanometer is an electromechanical instrument which is used for the detection of electric currents     through electric circuits. Being a sensitive instrument, Galvanometer can not be used for the     measurement of heavy currents. However we can measure very small currents by using galvanometer but     the primary purpose of galvanometer is the detection of electric current not the measurement of current.

Galvanometer works on the principle of conversion of electrical energy into mechanical energy. When a     current flows in a magnetic field it experiences a magnetic torque. If it is free to rotate under a     controlling torque, it rotates through an angle proportional to the current flowing through it.

here are five essential parts of a Galvanometer.
    1. A U-shaped permanent magnet with concave poles.
    2. Flat rectangular coil of thin enameled insulated wire ‘C’.
    3. A soft iron cylinder 'B'.
    4. A pointer or needle.
    5. A scale.

The flat rectangular coil of thin enameled insulated wire of suitable number of turns wound on a light     nonmetallic or aluminum frame is suspended between the cylindrically concave poles of magnet by a     thin phosphor bronze strip. One end of the wire of the coil is soldered to strip. The other end of the strip     fixed to the frame of the galvanometer and connected to an external terminal. It serves as one leas     current lead through which the current enters or leaves the coil. The other end of the wire of the coil is     soldered to a loose and soft spiral of wire connected to another external terminal. The soft spiral of a     wire serves as the other current lead. A soft-iron cylinder, coaxial with the pole pieces, is placed within     the frame of the coil and is fixed to the body of the galvanometer. In the space between it and the pole     pieces, where the coil moves freely, the soft iron cylinder makes the magnetic field stronger and radial     such that into whatever position the coil rotates, the magnetic field is always parallel to its plane.

When a current passes through the galvanometer coil, it experiences a magnetic deflecting torque,     which tends to rotate it from its rest position. As the coil rotates it produces a twist in the suspension     strip. The twist in the strip produces an electric restoring torque. The coil rotates until the elastic     restoring torque due to the strip does not equal and cancels the deflecting magnetic torque, then it     attains equilibrium and stops rotating any furthers.

Hope this helps.

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