Moving Coil Galvanometer

Moving Coil Galvanometer is an electromagnetic device that can measure current. It consists of permanent horseshoe magnets, pivoted spring, soft iron core, coil, non-metallic frame, scale, and pointer. Moving coil galvanometers is a device which can measure even low currents of few micro-amperes.

History

The deflection of a magnetic compass needle by the current in a wire was first described by Hans Oersted in 1820.

The phenomenon was studied not only for its own sake but also as a means of measuring electrical current. The earliest galvanometer was designed and reported by Johann Schweigger at the University of Halle on 16 September 1820. André-Marie Ampère also contributed to its development.

Early designs increased the effect of the magnetic field generated by the current by increasing the no. of turns of the wire.

The term “galvanometer,” was commonly used by 1836 and was derived from the surname of Italian electricity researcher Luigi Galvani, who in 1791 discovered that electric current would make a dead frog’s leg jerk.

Principle of Moving Coil Galvanometer: Torque acts on a current-carrying coil suspended in the uniform magnetic field. Due to this, the coil rotates. Hence, the deflection in the coil of a moving coil galvanometer is directly proportional to the current flowing in the coil.

Construction of Moving Coil Galvanometer

Moving Coil Galvanometer consists of a rectangular coil of a huge number of turns of thinly insulated copper wire wound over a light metallic frame.

The coil is suspended between the pole pieces of a horseshoe magnet by a fine phosphor – bronze strip from a movable torsion head to suspend the coil in a uniform radial magnetic field. The lower end of the coil is attached to a hairspring of phosphor bronze having only a small number of turns.

The other end of the spring is connected to a binding screw. A soft iron cylinder is placed symmetrically inside a coil.

The hemispherical magnetic poles produce a radial magnetic field in which the plane of the coil is parallel to the magnetic field at all positions.

A small plane mirror attached to the suspension wire which is used with a lamp and scale arrangement to measure the deflection of the coil.

Working of Moving Coil Galvanometer

Let, l, d – the length of the respective vertical and horizontal side of the coil in meters.
N – number of turns in the coil,
B – Flux density in the air gap, wb/m²
i – current through moving coil in Ampere
K – spring constant of suspension, Nm/rad
_{\(\bf{\theta_{F}}\)} – final steady-state deflection of moving coil in radiance

When the current flows through the coil, it experiences a torque which is expressed as
T_d = Force * Distance

The force on each side of the coil is given as
F = NBil

Hence deflecting torque becomes,
T_d = NBild
=> T_d = NBAi
Where,
  A = l * d

We can then write T_d as
T_d = Gi
and N, B, A are the constant of the galvanometer.

Displacement constant of the galvanometer is denoted as ‘G’ , and its value is = NBA or NBld

The controlling torque exerted by the suspension at deflection θ_F is
T_c = Kθ_F

For final steady deflection,
T_d = T_C
Kθ_F = Gi

Hence final steady deflection,
θ_F = Gi/K

For the small deflection angle, the deflection is expressed as the product of the radius(distance between the centre of mirror and light source) and the angle turned by the reflected beam. It is expressed as 1000 Χ 2θ_F = 2000 Gi / K in millimetre.

In the above equation, 1000 is the radius i.e distance between mirror and light source and 2θ_F is the angle turned because mirror rotates by θ_F and reflected beam rotates by 2θ_F.

Application of Moving Coil Galvanometer

By now, I hope you understood that a moving coil galvanometer is an instrument which is used to measure electric currents. Mainly it is used for following applications:

1. Detecting the current(i) in a circuit.
2. To convert it into Ammeter to measure current.
3. To convert it into a voltmeter to measure the potential difference.  
4. Detecting the direction of flow of current.
5. They are used in laser engraving, laser sintering, displays like monitors and televisions.  
6. They are used for controlling the head positioning servos in the hard drives and CD/DVD players and music players

Advantages and Disadvantages of Moving Coil Galvanometer

Advantages

• Does not get easily affected by strong magnetic fields.
• They have a high torque to weight ratio.
• High accuracy and very reliable.
• Galvanometer also has uniform scales.

Disadvantages

• It only measures Direct currents (DC) and does not measures Alternating current quantities can not be measured
• Temperature change causes a change in restoring torque.
• During severe stresses or due to factors like aging of the instrument Phosphor bronze fiber or Helical restoring spring are susceptible to damage.
• Changing Restoring Torque is not easy.

FAQs