Introduction to the Principle and Structure of the Galvanometer

High sensitivity mechanical indicating meter is used to detect weak electrical quantities. Galvanometer is used as a zero meter instrument in bridge circuits and potential difference measurements. It can also be used to measure weak electrical currents, voltages, charges, etc. There are mainly magnetic-electric amperemeter, photoelectric amplification amperemeter, impulse amperemeter, vibration amperemeter and oscillator.

Working principle of galvanometer

Galvanometer is a magnetic-electric instrument, which is made according to the principle that the current-carrying coil is deflected in the magnetic field. In a common meter, the coil is placed on a bearing, balanced by a spring wire, and indicated by a pointer. Due to friction in the bearing, the measured current cannot be too weak. Galvanometer working uses fine metal filaments instead of bearings to hang in the magnetic field. Because the filament is thin and long, the reaction torque is small. Therefore, when a weak current passes through the coil, it is sufficient to cause significant deflection. Therefore, the galvanometer is much more sensitive than a general current meter, and can measure microcurrents or microvoltages, such as photoelectric currents, physiological currents, thermoelectric potentials, etc. The first recording of nerve action potentials was accomplished with this type of instrument. Another use of the galvanometer is zero balancing, that is, judging whether the circuit is balanced based on whether the current flowing through the galvanometer is zero. It is widely used in DC bridges and potential difference measurements.

Structure of galvanometer

Galvanometer consists of three parts:

Magnetic field part

Permanent magnets generate a magnetic field, and the cylindrical soft iron core makes the magnetic field in the air gap radiate uniformly.

Deflection part

A rectangular coil that can rotate in the air gap and metal tension filaments pulled up and down from the coil can cause the coil to deflect with a small torque.

Reading part

The small mirror is fixed on the moving coil. It reflects the light beam from the light source onto the scale to form a pointer. When current flows through the moving coil, the moving coil is deflected, which causes the small mirror to turn over an angle and reflects the light beam to move along the scale. The distance from the mirror to the scale is the distance.

Because the galvanometer is very sensitive, the current cannot exceed 1μA. After the voltage is divided twice, it is only added to the galvanometer circuit after obtaining a very small voltage (usually less than 1mV). The first time uses a sliding variable resistor for voltage division, and the second time uses resistors of orders of magnitude. K2 is a reversing switch that can change the direction of the current through the galvanometer. K3 is a damping switch, and turning it on can short-circuit the galvanometer and stop the moving coil from vibrating.