Physics · Moving Charges And Magnetism · NEET
Principle: a current-carrying coil placed in a magnetic field experiences a torque (tau = NIAB). This magnetic torque rotates the coil. A phosphor-bronze spring provides an opposite restoring torque (tau = C phi) that grows as the coil turns. The coil stops where the two torques are equal: NIAB = C phi. So phi = (NAB / C) I, meaning the pointer's deflection is directly proportional to the current. That is why the scale can be marked in equal (linear) divisions.
In a normal (uniform) field the torque is tau = NIAB sin theta, so it depends on the angle theta between the coil and the field, and the scale would be non-linear (crowded). A radial field is made using a cylindrical soft iron core plus concave pole pieces. In a radial field the plane of the coil is always parallel to B, so the angle between the coil's area vector and B is always 90 degrees and sin theta = 1. Then tau = NIAB at every position, deflection stays proportional to current, and the scale is linear.
The cylindrical soft iron core does two jobs. First, it makes the field radial together with the concave (curved) pole pieces, so the field is always along the coil plane. Second, being soft iron it has high permeability, so it concentrates the magnetic field lines and makes B stronger. A stronger B increases the torque per unit current, which increases the galvanometer's sensitivity.
When current flows, the magnetic torque NIAB starts turning the coil. As the coil turns by angle phi, the spring twists and produces a restoring torque C phi that increases with phi. The coil keeps rotating until the restoring torque exactly equals the deflecting torque: C phi = NIAB. At that point net torque is zero, so the coil rests there. A larger current needs a larger phi to balance it, which is why the pointer shows a steady reading for each current.
The general torque on a coil in a magnetic field is tau = NIAB sin theta, where theta is the angle between the field B and the normal (area vector) of the coil. In a moving coil galvanometer the field is radial, so the coil plane is always parallel to B, theta = 90 degrees, and sin theta = 1. Only then does it simplify to tau = NIAB. Use NIAB sin theta for a general loop in a uniform field, and NIAB only for the radial-field galvanometer.
Current sensitivity of a moving coil galvanometer is 5 div/mA and its voltage sensitivity (angular deflection per unit voltage applied) is 20 div/V. The resistance of the galvanometer is:
Try the real previous-year questions from this chapter — each with the answer and a full solution.
It is a sensitive instrument that detects and measures small electric currents. A multi-turn coil sits in a magnetic field; current makes the coil rotate against a spring, and a pointer shows the deflection, which is proportional to the current.
At balance, deflecting torque equals restoring torque: NIAB = C phi. Rearranged, phi = (NAB / C) I. Here N is number of turns, A area, B radial field, C the spring's torsional constant, and phi the deflection. Deflection is directly proportional to current.
A galvanometer only handles tiny currents (microampere to milliampere) and has a fixed resistance. Large currents would damage the coil. To measure real currents you add a small parallel shunt (ammeter) and to measure voltage you add a large series resistance (voltmeter).
Sensitivity phi/I = NBA/C increases with more turns N, a larger coil area A, a stronger radial field B, and a weaker (smaller) spring constant C. That is why the soft iron core (raises B) and many fine turns are used.
