CBSE Class 12 Physics Electrical Instrument Theory and Examples

CBSE Class 12 Physics Electrical Instrument Theory and Examples. Please refer to the examination notes which you can use for preparing and revising for exams. These notes will help you to revise the concepts quickly and get good marks.

1. MOVING–COIL METERS

1.1 GALVANOMETER
(a) The instrument used to measure strength of current, by measuring the deflection of the coil due to torque produced by a magnetic field, is known as galvanometer.
Types of galvanometer :
(I) Moving magnet type In this type of galvanometer, magnet is movable coil is stationary.
(II) Moving coil type In this type of galvanometer, the coil is movable and and magnet is stationary.
(a) Suspended coil type galvanometer

(i) In this the coil is suspended by a fine phosphor bronze fibre
(ii) In this the deflection of coil is measured by lamp and scale arrangement
(iii) Its sensitivity is high [current of order 10^–9 amp. can be measured by it

(i) In this the coil is pivoted on a pivot about which it is free to rotates.
(ii) In this the deflection of coil is determined by a pointer rotating on a circular scale.
(iii) Its sensitivity is low [current of order 10^–6 amp. can be measured]

1.2 MOVING COIL TYPE GALVANOMETER
(a) Principle: When a current carrying coil is placed in magnetic field, it experiences a couple and the coil rotates.
(b) The torque of this couple is given by τ = NiAB sin α Where N – no. of turns in the coil 

i – current through coil
B– intensity of magnetic field
α – angle between magnetic field and plane of the coil.
(c) For radial magnetic field α = 90º
∴τ = NiAB
(d) In the state of equilibrium of coil deflecting torque = restoring torque ⇒ NiAB = C θ Where C is restoring torque per unit deflection, (C depends on elastic properties of spring)
or i =C/NAB θ or i α θ

The current is proportional to deflection. The constant  C/NAB  is called galvanometer constant.

(e) Sensitivity of galvanometer–
(i) The deflection of galvanometer coil per unit current is defined as the sensitivity of galvanometer.
(ii) S_i =θ/i 
(iii) The deflection obtained by unit current is S_i
(f) The galvanometer which gives more deflection for a small current is more sensitive. To increase sensitivity the values of N, A, B must be large and the value of C must be small.
(g) Figure of merit (K)
(i) The reciprocal of current sensitivity is defined as the figure of merit of galvanometer.
(ii) The current required for unit deflection in galvanometer is known as figure of merit(K)
(ii) S_i = 1/K or K =S_i

(h) Voltage sensitivity (S_v)
(i) The deflection of galvanometer coil per unit voltage is defined as voltage sensitivity of galvanometer.
(ii) S_v =Q/A= Q/iRG=NAB/CR_G, where R_G isresistance of galvanometer coil.

Moving-coil meters
Ex. 1 Which is more sensitive, either suspended type galvanometer or pivoted type galvanometer ?
Ans. Suspended type galvanometer. The sensitivity of suspended type galvanometer is of order 10^–9 A., while that of pivoted type is of 10^–6A.

Ex. 2 In your practical classes, which type of galvanometer do you use ?
Ans. Pivoted–coil (or Weston type) Galvanometer. In the suspended coil galvanometer the suspension strip is very delicate and may be broken by a small jerk. Therefore this galvanometer can not be taken from one place to another. This difficulty has been removed by westone galvanometer.

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