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Moving Coil Galvanometer Class 12

Ever wondered how the utility company detects how much power is used each month? The galvanometer is an instrument used to determine the presence, direction, and strength of an electric current in a conductor. Here we will study about the Galvanometer, moving Coil Galvanometer class 12 and its types. These are instruments used for detection and measurement of small currents.   The moving coil galvanometer work on the principle that when a current carrying coil is placed in a magnetic field it experiences a torque.   The different type of moving coil galvanometers are (a) Pivoted Galvanometer : It consists of a coil of fine insulated wire wound on a metallic frame. The coil is mounted on two jewelled pivots and is symmetrically placed between cylindrical pole pieces of a strong permanent horse-shoe magnet. (b) Dead beat Galvanometer : Here coil is wound over the metallic frame to make it dead beat. On passing current the galvanometer shows a steady deflection without any oscillation. The damping is produced by eddy currents. (c) Ballistic Galvanometer : This is used for measurement of charge. Here coil is wound on an insulating frame and oscillates on passing current. In moving coil galvanometer the deflection produced is proportional to current flowing through galvanometer i.e. they have linear scale of measurement. In equilibrium deflecting torque = restoring torque i.e. NIAB $=\mathrm{C} \theta \quad$ or $\quad I=\left(\frac{C}{N A B}\right) \theta \quad$ or $I=K \theta$ where $\mathrm{K}$ is galvanometer constant.  

Current Sensitivity

This is defined as the deflection produced in galvanometer when a unit current flows through it. Current sensitivity $CS =\frac{\theta}{ I }=\frac{ NAB }{ C }$ radian/ampere or division/ampere (a) The sensitivity can be increased by increasing number of turns in coil (N), area of cross-section of coil (A), magnetic field B and decreasing torsional constant (C). (b) The reciprocal of current sensitivity is called figure of merit. Figure of merit $FM =\frac{ I }{\theta}=\frac{ C }{ NAB }$     Voltage Sensitivity  This is defined as the deflection produced in galvanometer when a unit voltage is applied across its terminals. Voltage sensitivity $VS =\frac{\theta}{ V }=\frac{\theta}{ IR }=\frac{ NAB }{ CR }$ division/volt, where R is resistance of coil.
  • Shunting a galvanometer reduces its current sensitivity.
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