We have discussed about currents and voltages in the previous sections and in many instances, we are required to measure them. But how do we perform the measurements? We have read about voltmeter that is used to measure the voltage across two points in a circuit, or an ammeter that is used to measure the current in a circuit. A moving coil galvanometer is one such device. In this section, we will learn about a moving coil galvanometer that is used to measure the current in a circuit.
In the figure shown below, we can see a moving coil galvanometer.
A galvanometer is a device consisting of a coil that is free to rotate about an axis that is fixed, such that a uniform magnetic field acts radially over the coil throughout the process. The system consists of a soft iron core that increases the strength of the field. When current is made to flow through the coil, it experiences a torque due to this field, which mathematically can be given as,
Where, N is the number of turns of the coil, I is the current flowing through the coil.
In the above expression of torque, sinθ has a value equal to 1, since the field is radial. We see that the magnetic torque tends to rotate the coil and a spring attached to the system provides a counter torque that balances this force, thus resulting in a steady angular deflection φ. In the equilibrium position, the counter force applied by the spring equals the force exerted by the magnetic torque. Thus,
Here, k is the torsional constant of the spring attached, which is given by the restoring torque acting per unit angle of the twist. The spring is attached further to a pointer, that indicates the angular deflection and is given by,
We notice that, for a given galvanometer, the value of (NAB/k) is constant and the deflection is thus proportional to the amount of current flowing through the circuit. Thus, a galvanometer is capable of indicating even small values of current in the circuit because of which it is also used to detect if any amount of current is flowing in the circuit or not, quite similar to a Wheatstone’s bridge. Here, when no current is flowing through the circuit, the pointer remains undeflected, that is, at the mean position and depending upon the direction of the current in the circuit, the pointer deflects to the right or to the left.
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