In this topic, you study the Construction, Diagram, Derivation & Working of galvanometer.
A galvanometer is the basic electrical instrument, which is used for detection of small currents (or voltages) and sometimes for measuring their magnitude also. When a current carrying conductor or coil is placed in a magnetic field, a mechanical force acts or the conductor (or coil), which tends to move in a direction decided by Fleming’s left hand rule. This is the principle involved in galvanometers. The galvanometer can be converted into ammeter and voltmeter to measure current and voltage respectively. D’Arsonval galvanometer is the most popular and widely used galvanometer for detection of currents/voltages and also for the measurement purposes.
Construction (Fig. 3.1).
This is a moving coil permanent magnet type instrument. The coil of many turns wound on a metal former is suspended, so as to move about its vertical axis. The coil moves in the air gap provided between the pole pieces. The air gap is about 1.5 mm. As the coil is wound on a non-magnetic (usually aluminium) former, the damping torque is provided by the eddy currents induced in the former during motion of the coil. This damping is very efficient and the instrument is dead beat. The damping can also be provided by connecting a low resistance across the galvanometer and the damping can be obtained by adjusting the value of the resistance. The coil is suspended by a phosphor bronze filament which also acts as a lead for the coil. The other lead is a flexible spiral filament at the bottom of the coil. These filaments are also called “upper” and “lower” suspensions.
The poles of the magnet are usually cylindrical in shape. By having cylindrical poles, the length of air gap is reduced so that the amount of flux linking with the coil is increased, this increases sensitivity of the instrument and the flux between the poles becomes radical. Moreover in a radial field, the deflection of the coil is directly proportional to the current in the coil and therefore a uniform scale is obtained.
Fig. 3.1. D’Arsonwal Galvanometer.
Working : When the current to be measured is made to flow through the coil a deflecting torque proportional to the product of flux density, current and dimensions of the coil makes the coil to rotate about its vertical axis. Now the suspension filaments produce controlling force and the pointer stops when the deflecting and controlling torques equal each other. As the deflecting torque is directly proportional to the current, the amount of deflection of the coil indicates the magnitude of current on calibrated scale. On few galvanometers, a mirror is attached to the moving system Fig. 3.2. A light beam is reflected from the mirror on to a glass scale. As the coil is deflected, the light beam moves over the scale.
Fig. 3.2. Mirror Arrangement.
The scale is about one (or half a) meter away from the instrument. This provides a high sensitivity of the galvanometer. The upper and lower suspensions are very weak and hence the instrument is not very strong mechanically, so the galvanometer needs to be handled carefully. A torsion head is provided for adjusting the position of coil and also for zero setting.
Deflecting Torque Equation (Fig. 3.3).
Let, dimensions of the coil — L x D
No. of turns in the coil N
Flux density in the air gap B
Current through the coil I
The angle between coil and the magnetic fields a
Hence, Force on each side of coil — NBIL sin ct
As for radial field 90
The expression becomes NBIL
Coil
s
Fig. 3.3. Torque Equation.
Deflecting torque T d Force x distance
= NBILD N-m.
As N, B, L and D are constants for the galvanometer,
G.l. (NBLD-G)
Note : See Fig. 3.4. If deflection is measured on a scale kept I m away; Then if deflection angle is θ, the mirror also turns through 0 and the deflection beam turns
Fig. 3.4. Measurement of Deflection