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Current is rate of flow of electron in a circuit. It is measured in the unit of the Ampere, also known as "Amp," (A). In order to measure the current flowing through a circuit, ammeter is used. There are two basic types of ammeter namely, the moving coil ammeter and moving iron or moving magnet ammeter.
Moving coil ammeter:
Structure
The moving coil type has a linear scale (i.e. divisions are equally spaced). The moving coil ammeter has three basic parts: a permanent magnet, a fiat coil pivoted at right angles to the magnet`s field and carrying the instrument`s pointer, and a coil spring. The moving coil and pointer assembly are pivoted on jewelled bearings, and their rotation is held in check by the coil spring.
Working
When current is passed through the coil, magnetic field is set up in the coil, which opposes the field due to permanent magnet. As a result of this, a force is generated which makes the coil and pointer rotate. Without the retaining coil spring, rotation of the coil continues until the two magnets stop opposing each other. However, the coil spring opposes the change in its state caused due to rotation. The resistance by the coil to get back to its original state depends upon the strength of the force twisting it. Thus, every position on the scale corresponds to a particular force and hence to a particular current.
Generally, an ammeter can measures direct current up to a fixed maximum known as full scale deflection - fsd. This can vary from as low as 25 microamperes in very sensitive instruments to higher values. Higher ranges can be added by connecting shunt resistors across the coil. These shunt resistors can carry most of the current, allowing only a small proportion to flow through the coil.
Accuracy of such instruments is frequently within 1 or 200 of fsd, but much higher accuracies are available in laboratory instruments.
Moving iron or moving magnet ammeter:
Structure
Moving iron or moving magnet meters have a nonlinear scale. Moving iron ammeter is a simpler and cheaper alternative to moving oil ammeter.
In moving iron ammeter, the pointer is fixed to a spring or counterbalance weight, and to a piece of soft iron. In addition, there is another piece of iron fixed near the first. In order to measure alternating currents, rectifiers (which turn AC into DC) have to be added to the shunt circuits.
Working
In Moving iron ammeters, the electromagnetic force of a fixed coil of wire causes movement of piece of iron. This type of meter responds to both direct and alternating currents (as opposed to the moving coil ammeter, which works on direct current only).
Nowadays, digital device like ADC is used to measure flow of current through any circuit. The ADC is read by a microcomputer that performs the calculations to display the current through the resistor
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