A pivoted coil type galvanometer is mostly used in laboratory where a very sensitive galvanometer is not required.
Construction:
The construction of a pivoted type galvanometer is shown in below figure. It consists of a rectangular coil of thin insulated copper wire wound on an aluminium frame. The coil is free to rotate on a vertical axle between the cylindrical pole pieces of a permanent horse–shoe magnet. The axle is pivoted at the two ends on jewelled bearings. The ends of the coil are connected to the springs, which serve to bring the coil back from its deflected position when current is switched off and also serve as leads of the terminals T1 and T2. A soft iron core is placed at the centre of the coil (without touching it) to make the magnetic field radial and to intensify the magnetic field.
The deflection of the coil is directly read on a scale with the help of a pointer attached at the top of the coil. The scale is graduated on either side with the centre marked as zero. It is therefore also called the centre–zero galvanometer. The graduations on the scale are equispaced. The pivoted type galvanometer is portable and convenient to use.
Working of a moving coil galvanometer:
The suspended coil and pivoted type galvanometer differs partially in their construction but their working is almost the same. Consider a current flowing in the coil of galvanometer in direction ABCDA as shown in below figure.
In the arms BC and DA, the current is parallel to the magnetic field, therefore no force acts on these arms.
In the arm AB, current is flowing from A to B in the downward direction, therefore according to Fleming's left hand rule a force acts on it in a direction perpendicular to the plane of paper outwards.
On the other hand, current in arm CD is flowing upwards from C to D and therefore the force of same magnitude acts on this arm, perpendicular to the plane of paper inwards.
Thus two equal and opposite forces on the arms AB and CD form a deflecting couple which rotates the coil such that the arm AB moves out and the arm CD goes in.
As the coil rotates, the suspension strip gets twisted and opposes the rotation of coil (in pivoted type, springs are twisted) and a restoring couple is produced.The opposing couple produced in the suspension strip is called the restoring couple. When the two couples balance each other, the coil stops. Thus in equilibrium, the deflecting couple is equal and opposite to the restoring couple.
The rotation of coil depends on the magnitude of deflecting couple which is directly proportional to the strength of current. Therefore, the deflection of coil is a measure of current passed in the coil.
If the direction of current is reversed (by changing the polarity at the terminals T1 and T2), the coil gets deflected in opposite direction.
Uses of a moving coil galvanometer:
- To detect the presence of current in an electric circuit.
- To detect the direction of flow of current in an electric circuit.
- To convert it into an ammeter so as to measure current in an electric circuit.
- To convert it into a voltmeter so as to measure the potentical difference between two points of an electric circuit.