A galvanometer is a device that is used for detecting the presence of small currents and voltages. It is also used for measuring their magnitude. The galvanometer is mainly used in the bridges where they indicate the null deflection or zero current.
Construction of the Galvanometer
There are some main parts of the galvanometer. The moving coil, suspension, and the permanent magnet are some of the parts of the galvanometer.
Moving Coil – The moving coil carried the current in the galvanometer. It is rectangular or circular shaped. It has many turns of copper wire. The coil is freely moved about the vertical axis of its symmetry. It is moved between the poles of a permanent magnet. The iron core provides the low reluctance path for the flux. It hence provides a strong magnetic field to move in.
Suspension – A coil is suspended by using a flat ribbon. It carries the current. The other current-carrying coil is in the lower suspension. Its torque effect is very negligible. The mechanical strength of the wire is not very strong, therefore the galvanometers are handled very carefully without any jerks.
Mirror – The suspension has a small mirror. It casts a beam of light. The beam of light is placed on the scale. It is used to measure the deflection.
Applications of Galvanometer
The galvanometer has the following applications.
It is used for detecting the direction of current flows in the circuit.
It is used for measuring the magnitude of the current.
The voltage between any two points of the circuit is determined through a galvanometer.
Working of Galvanometer
Let, l, d be the length of the vertical and horizontal side of the coil.
N is considered as the number of turns in the coil,
B be the Flux density in the air gap, wb/m2
i is the current through moving coil in Ampere
K is the spring constant of suspension, Nm/rad
θf is the final steady-state deflection of moving coil in radiance
The force on each side of the coil is given and written as,
deflecting torque is,
N, B, A are the constant in the galvanometer.
The G is called the displacement constant, and its value is equal to NBA = NBld.
This above written-equation shows us that when the mirror turns through an angle the reflected beam turns through an angled double of it.
Conversion of Galvanometer into an Ammeter
The galvanometer is also used as an ammeter. It is done by connecting the low resistance wire in parallel with the galvanometer. The potential difference between the voltage and the shunt resistance is kept equal.
Thus, the shunt resistance is written as,
The value of the shunt current is very small as compared to the supply current.
Moving Coil Galvanometer
Moving coil galvanometer is a device that measures small values of current. It consists of a coil, soft iron core, pivoted spring, non-metallic frame, scale, and a pointer.
Principle of the Moving Coil Galvanometer
Torque always acts on a current-carrying coil suspended in the uniform magnetic field. The coil rotates. The deflection in the coil due to current flowing in a moving coil galvanometer is directly proportional to the current flowing in the coil.
Construction of Moving Coil Galvanometer
It consists of a rectangular coil of a large 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. The lower end of the coil is connected to a hairspring of phosphor bronze having only a few turns. The other end of the spring is always connected to a binding screw. A small plane mirror attached to the suspension wire is used along with a lamp and scale arrangement to measure the deflection of the coil.
A Tangent galvanometer is used for measuring current. It works on the tangent law. A magnetic needle is suspended at a point where there are two crossed fields at right angles.
It consists of a circular coil made up of wire wound over a non-magnetic frame. The vertical frame is mounted on a horizontal circular table provided with three screws. The vertical frame can be rotated about its diameter.
The compass box has a small pivoted magnet. A thin long aluminum pointer is fixed at right angles to it. The scale consists of only four quadrants. The compass box is supported such that the center of the magnetic needle coincides with the center of the coil. The magnetic field at the center of the coil is uniform, a small magnetic needle is used so that it remains in a uniform field.
The ballistic galvanometer is basically designed to deflect its needle. It is done in a way that is proportional to the total charge passing through its moving coil or to a voltage pulse in a short duration. A conventional galvanometer may also be used as a ballistic type, but it has a smaller torque and higher inertia in the coil. A moving-coil galvanometer helps us to indicate the presence of an electric charge by the single impulse imparted to the coil or by a sudden brief current, the quantity of electricity that passes being proportionate to the first deflection of the coil.