Galvanometer Moving-coil MeterThe simple galvanometer or current indicating instrument was designed by "Johann Schweigger".(German Scientist) in 19th Century. There are 2 types of galvanometer:-
A) Moving Magnet Galvanometer:-
Instruments of the moving magnet type with various modifications to make them more sensitive were in general use during the 19th century. They did, however, have certain disadvantages. Not only were they effected by stray magnetic fields setup by nearby motors and power cables, but also they always had to be set up in one particular direction owing to the fact that the earth's field acted as the control field. For these reasons they become obsolete and have been replaced by "Moving Coil Galvanometer".
B) Moving Coil Galvanometer:-
Moving Coil Galvanometer first designed by "Sturgeon" in 1836 later the same principle was adopted by lord kelvin as a means of detecting small electric currents sent through the first submarine telegraph cables. The design of modern moving coil galvanometer is based on improvements in construction introduced by Despretz and D' Arsonval.
Its underlying principle is the fact that when a current flows in a rectangular coil placed in a magnetic field it experiences a magnetic torque. If it is free to rotate under controlling torque, it rotates through and angle proportional to the current flowing through it. The rotation or deflection thus indicates a current through it and the amount of deflection indicates the amount of current in the coil. When the polarity is connected to correctly the pointer will read up-scale, to the right, the incorrect polarity forces the pointer off-scale, to the left.
Construction of Moving Coil Galvanometer:-
The essential parts of moving coil galvanometer are:-
1) A U-Shaped permenant magnet with cylindrical concave pole faces.
2) A flat coild of thin enamel insulated wire (usually rectangular).
3) A soft iron cylinder.
4) A pointer.
5) A scale.
Working of Galvanometer:-
The flat rectangular coil of thin enamel insulated wire of suitable number of turns wound on a light non metallic (or aluminium) frame is suspended b/w the cylindrically concave pole pieces of the permenant U-shaped magnet by a thin phosphor bronze strip. One end of the wire of the coil is soldered to the strip. The other end of the strip is fixed to the frame of the galvanometer and the connected to an external terminal. It serves as one current lead through which the current enters or leaves the coil. The other end of the wire of the coil is soldered to a loose and soft spiral of wire connected to another external terminal. The soft spiral of wire serves as the other current lead. A soft - iron cylinder, coaxial with the pole pieces, is placed with in the frame of the coil moved freely, the soft iron cylinder makes the magnetic field stronger and radial such that into whatever position the coil rotates, the magnetic field is always parallel to its plane.
When a current passes through the galvanometer coil it experiences a magnetic deflecting torque which tends to rotate it from its rest position. As the coild rotates it produces a twist in the suspension strip. The twist in the strip produces an elastic restoring torque. The coil rotates until the elastic restoring torque due to the strip does not equal and cancel the deflecting magnetic torque and then it attain equlibrium and stop rotating any further. The deflecting magnetic torque was derived as:
Deflecting magnetic torque = BINA cosØ
B = Strength of the magnetic field
I = Current in the coil
A = Area of the coil
N = Number of turns in the coil
Ø = The angle of deflection of the coil
The restoring elastic torque is proportional to the angle of twist of the suspension strip provided it obeys Hooke's Law. Thus restoring elastic torque = Cß where ß is the angle of twist of the suspension strip (ß is different from Ø but proportional) and c is the torque per unit twist of the suspension strip for equilibrium.
Deflecting magnetic torque = Restoring elastic torque
BINA cosØ = cß
I= c ß
If the magnetic field were uniform (as with flat pole pieces) Ø would continously increase with ß and cosØ factor would not be constant then the current would not be proportional to ß and the scale of the galvanometer not linear. However, due to the radial magnetic field the plane of the coil is always parallel to the field irrespective of the position the coild rotate, so , Ø the angle between the plane of the coil and the direction of the field is always zero, hence cosØ = 1 that is constant as are B, A and N.
Thus the current through the coil is directly proportional to the angle of twist of the suspension (or deflection) ß, giving a linear scale.
The coil instead of being suspended by a strip is pivoted between two jewelled bearings. The controlling (restoring) torque is provided by two hair springs one on either side of the coil and curling in the opposite sense. The two ends of the coil are connected one to each spring. The hair springs thus also serve as current leads to the coil. A light aluminium pointer is fixed to the coil which moves over a calibrated circular scale with equal divisions which measures the deflection (in divisions) or current (in micro amperes) directly.