galvanometer - Operation, Types, Uses

An instrument for measuring small electrical currents. The moving coil galvanometer consists of an indicating needle or mirror attached to a coiled wire suspended in a magnetic field. The coil rotates when a current passes through it. The angle through which it rotates (indicated by the deflection of the needle or by a beam of light reflected from the mirror) is used to measure the current. Other models are ballistic galvanometers and moving-magnet instruments.

(Discuss)

A galvanometer is an instrument for detecting and measuring electric current. It is an electromechanical transducer that produces a rotary deflection, through a limited arc, in response to electric current flowing through its coil.

Operation

The most familiar use is as an analog measuring instrument, often called a meter. It is used to measure the direct current (flow of electric charges) through an electric circuit. The coil is attached to a thin pointer that traverses a calibrated scale. A tiny spring pulls the coil and pointer to the zero position.

When a direct current (DC) flows through the coil, the coil generates a magnetic field. The coil pivots, pushing against the spring, and moving the pointer. The hand points at a scale indicating the electric current. A useful meter generally contains some provision for damping the mechanical resonance of the moving coil and pointer so that the pointer position smoothly tracks the current without excess vibration.

The basic sensitivity of a meter might be, for instance, 100 microamperes full scale (with a voltage drop of, say, 50 millivolts at full current). The use of current dividers, often called shunts, allows a meter to be calibrated to measure larger currents. A meter can be calibrated as a DC voltmeter if the resistance of the coil is known by calculating the voltage required to generate a full scale current. This is generally done by placing a resistor in series with the meter coil. A meter can be used to read resistance by placing it in series with a known voltage (a battery) and an adjustable resistor. In a preparatory step, the circuit is completed and the resistor adjusted to produce full scale deflection. When an unknown resistor is placed in series in the circuit the current will be less than full scale and an appropriately calibrated scale can display the value of the previously-unknown resistor.

Because the pointer of the meter is usually a small distance above the scale of the meter, parallax error can occur when the operator attempts to read the scale line that "lines up" with the pointer. To counter this, some meters include a mirror along the markings of the principal scale. The accuracy of the reading from a mirrored scale is improved by moving the head while reading the scale so that the pointer and the reflection of the pointer are aligned;

Types

Extremely sensitive measuring equipment once used mirror galvanometers that substituted a mirror for the pointer. The moving beam of light could also be used to make a record on a moving photographic film, producing a graph of current versus time, in a device called an oscillograph.

Galvanometer mechanisms are used to position the pens of analog chart recorders such as used for making an electrocardiogram. Strip chart recorders with galvanometer driven pens might have a full scale frequency response of 100 Hz and several centimeters deflection.

Uses

Mirror galvanometer systems are used as beam positioning elements in laser optical systems. There are, for instance, self contained digital measuring systems, called digital panel meters (DPMs), available to replace most traditional analog meter functions.