The measurement of electric current was not so precise and accurate during earlier days. Electric current is measured by the intensity of the field of a magnet. After the invention of galvanometers, the measurement of electric current became easy. Galvanometers are instruments made to measure electric current.

Working principle of a galvanometer

The working principle of the galvanometer is based on the theories and principles of conservation of energy from mechanical to electrical. A coil that carries the current placed in a magnetic field experiences torque. Due to the torque, the coil present in the galvanometer rotates. The deflection of the current-carrying coil is directly proportional to the flow of current.

The working of a galvanometer

A Galvanometer contains a current-carrying coil made of fine copper. A soft iron core of cylindrical shape is placed inside the coil to increase the strength of the magnet’s field. The lower part of the coil is connected to a bronze-phosphor spring. A magnetic field can be produced when the current flows through the coil and the coil experiences a torque. As a result of this torque, the coil deflects. The phosphor-bronze spring also twists. The spring produces a counter torque, resulting in the proper angular deflection of the coil. The deflection in the galvanometer is measured as the value on the scale shown by the pointer linked to the suspension wire. The deflection occurring in the galvanometer is directly proportional to the current flowing through the galvanometer.

Uses of a galvanometer

Galvanometers are highly sensitive instruments as they detect current in any circuit. There are many uses of the galvanometer in physics.

In earlier days, galvanometers were mostly used for detecting errors in telecommunication cables.

• A galvanometer detects the direction of the flow of the electric current in the circuit.
• It determines the null point.
• It measures the amount of current passing through.
• It can measure the voltage between two points in the circuit.
• Galvanometer has its application in various control systems, laser TVs, laser displays, etc.
• Galvanometers are applied on CD/DVD players and hard discs for controlling the head server positions. 
• Galvanometers are also used in film cameras to get photoresistor readings in the metering mechanisms.

Types of galvanometers

• Thermo-galvanometer: They are used to measure small currents. There is no self-induction or capacity in it, and hence it can be used on any circuit.
• String galvanometer: Earlier, they were used to detect small electrical currents of the human heart and gave the first electrocardiogram.
• Mirror galvanometer: Like all other galvanometers, a mirror galvanometer also detects and measures the electric current. The only difference is that mirror galvanometers use mirrors instead of needles. They are used in scientific instruments, recording seismometres, and submarine cables.
• Astatic galvanometer: They have two magnetic needles placed parallel to each other, and the magnetic poles are reversed. One needle has no magnetic influence and is balanced on a thread. The lower needle is placed on the inside of the coil. 
• Vibration galvanometer: It detects current with the frequency of the resonance.

Merits and demerits of galvanometers 

Every instrument has its positives as well as negatives. Here are a few merits and demerits of galvanometers.

Merits

• Galvanometers are highly sensitive as they show even small deflections for the current.
• They are not easily affected by the stray magnetic field.
• The scale is uniform.
• The ratio between the torque and weight is very high.
• Galvanometers are highly accurate as well as reliable.

Demerits

• Galvanometers can only measure direct currents.
• Overloading the galvanometers leads to their damage. 
• The restoring torque cannot be changed easily. 
• If there is a temperature change, there will be a change in the restoring force also.
• Factors like the age of the instrument, the magnetic capacity of the magnets, damage in the spring etc., influence galvanometers and may cause errors in these factors.

Conclusion

Galvanometers are the devices used to detect electric current in a circuit. The working principle of galvanometers is based on the conservation of energy from mechanical to electrical. This leads to deflections in the coil and gives the reading of the current. Galvanometers that we used earlier were so different from the calculators that are prominent today. A galvanometer has both positive and negative terminals. They can detect the current only in a direct circuit. Galvanometers are of different types: thermo-galvanometers, vibration galvanometers, astatic galvanometers, mirror galvanometers, string galvanometers, etc. Galvanometers are influenced by the number of turns in the coil, magnetic field, etc.