MCQ's On Galvanometer|Galavnometer Questions & Answers

Contents

What is a Galvanometer?

A galvanometer is an instrument used for detecting the presence of small currents or voltages in a circuit or for measuring their magnitudes.

Galvanometers find their principal application in bridge and potentiometer measurements where their function is to indicate zero current.

Therefore, a galvanometer in addition to being sensitive, should have a stable zero, a short periodic time and nearly critical damping.

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MCQ’s on Galvanometer

1. In a d’Arsonval galvanometer, an iron core is usually used between the permanent magnet pole faces. This is used so that

(a) flux density in the air gap becomes high thereby a large deflecting torque is produced
(b)the effect of stray magnetic fields is reduced

2. Sometimes, the d’Arsonval galvanometers, do not use ferromagnetic cores between poles of the permanent magnet. In this case

(a) the flux density becomes smaller resulting in low deflecting torque
(b)the dimensions of the moving coil can be made smaller thereby reducing the moment of inertia
(c)the magnetic field may not be radial resulting in a non-uniform scale even if spring control is used
(d)all the above.

3. A d’Arsonval galvanometer uses a light and scale arrangement. The light source is placed 1 m away from the moving system of galvanometer. The arrangement uses a circular scale calibrated in mm. The deflection indicated by the scale is

(a) 1000 mm
(b) 2000 mm
(c) 500 mm
(d) none of the above.

If a current of 1 pA is passed through the coil. The spring stiffness is 2xl0“6 Nm/rad and the displacement constant is 2 Nm/A.

4. The time period of free oscillations in a galvanometer having a relative damping of 0.6 is 2 s. The frequency of damped oscillations is :

(a) 0.5 rad/s
(b) 0.3 rad/s
(c) 0.4 rad/s
(d) none of the above.

5. The relative damping in a galvanometer is 0.8. Its logarithmic decrement is approximately

(a) 0.48
(b) 1.25
(c) 4.19
(d) -4.19.

6. In a critically damped galvanometer the deflection at a time 0.9 times the time of free oscillations after a current is passed through the moving coil is approximately

(a) 0.986 times the final deflection
(b) 0.901 times the final deflection
(c) 0.866 times the final deflection
(d) none of the above

7. The resistance required for critical damping in a circuit is 1000 Q. The galvanometer circuit has a resistance of 800 Q. Is the galvanometer circuit

(a) underdamped (b) undamped (c) overdamped (d) none of the above

8. A galvanometer has a ratio of 0.9 for damped frequency oscillations to undamped frequency of oscillations. Suppose moment of inertia, stiffness constant and damping constant are made twice their original value, what would be the new ratio of damped frequency oscillations to undamped frequency oscillations,

(a) 0.9 (b) 1.11 (c)4 (d)2

9. A current of 2 pA is passed through the moving coil of an undamped d’Arsonval galvanometer which has a displacement constant of 2 Nm/A and a control constant of 10 x 10-6 Nm/rad. The moving oscillates with an amplitude of :

(a) 0.2 rad
(b)0.4 rad
(c) 0.8 rad
(d) none of the above.

10. If the damping in a d’Arsonval galvanometer is only due to electromagnetic effects, the resistance required for critical damping is

where G = displacement constant ; Nm/A, K = control constant; Nm/rad and J = inertia constant; kg-m^2

11. A d’Arsonval galvanometer uses a lamp and scale arrangement. Its current sensitivity is 250 mm/pA. If the resistance of the coil is 100 Q and the external resistance required for critical damping is 900 Q (which is connected in the circuit), the voltage sensitivity is

(a) 0.25 mm/micro V
(b) 0.25 mm/V

(d) 250 mm/micro V.

12. Ayrton shunt is used in d’Arsonval galvanometers so as to limit the current in the galvanometer coil to its maximum permissible value. The relative value of current through the galvanometer coil and the shunt

(a) depends upon the value of resistance of galvanometer coil only
(b) depends upon the values of resistance of galvanometer coil and the shunt
(c) does not depend upon the value of resistance of galvanometer coil
(d) none of the above.

13. A Ballistic galvanometer should be designed with

(a) a large period of natural oscillations and a negligible damping constant
(b) a small period of natural oscillations and a high damping constant
(c) a large period of natural oscillations and a high damping factor
(d) a small period of natural oscillations and a low damping factor.

14. In a flux meter

(a) the controlling torque is produced by weights attached to moving coil
(b) the controlling torque is produced by springs

15. In an unshunted flux meter, the sensitivity is dependent upon

(a) resistance of moving coil
(b) resistance of search coil
(c) resistance of both moving coil and search coil

(d) none of the above.

16. A vibration galvanometer is to be tuned to a frequency of 50 Hz. The ratio of its control constant to inertia constant should be

(a) 98696
(b) 2500
(c) 10.132 x 10^-6
(d)none of the above.

17. A vibration galvanometer is tuned

(a) by changing the length and tension of vibrating coil
(b) by attaching weight to the vibrating coil
(c) by changing its damping constant
(d) all the above

18.A Duddell’s oscillograph can be used for frequencies

(a) upto 50 Hz
(b) upto 500 Hz
(c) above 500 Hz
(d) upto 10 kHz.

19.A Duddell’s oscillograph will give no amplitude distortion and phase displacement if its

(a) moment of inertia and stiffness constant are zero
(ft) stiffness constant and damping factor are zero
(c) moment of inertia and damping factor are zero (d) all the above.

In a Duddell’s oscilloscope the phase displacements of fundamental and 13th harmonic are calculated to be 3° and 36° respectively. The oscillograph will show the 13th harmonic to be (a) 39° ahead of its true position (b) 38° ahead of its true position
(c) 3° ahead of its true position
(d) 3° behind its true position.