Ammeter
An ammeter is an instrument that measures electric current, usually in units of amperes (or milliamps or microamps when very small currents are involved). Both the magnitude of direct current (DC) and alternating current (AC) can be measured, though with ammeters of different types. Due to differences in design, an ammeter is not as sensitive as a galvanometer.
There are two basic types of ammeters. The moving coil ammeter has a scale with equally spaced divisions; the moving iron or moving magnet ammeter has a nonlinear scale. Both versions use a principle discovered by Hans Christian Oersted in 1820, which holds that an electric current in a wire produces a magnetic field. The magnetic field exerts a force on a coil, making it turn. A pointer that is attached to the coil or magnet also moves, allowing the measurement to be read off the scale.
The moving coil ammeter has three basic parts: a permanent magnet, a coil, and a coil spring. When current is passed through the coil it creates a magnetic field that is opposite that of the permanent magnet, causing the coil, and its attached pointer, to pivot. The coil spring not only limits the distance the coil can rotate, but the amount of tension it exerts depends on the strength of the force turning the coil.
There is a fixed maximum of DC, called a full scale deflection (fsd), that an ammeter can measure. To obtain high values, a shunt resistor is attached to each end of the coil in a DC ammeter, causing a part of the current to bypass the meter movement and preventing an overload. To determine the true amperage of the current, it is then necessary to multiply the amperage indicated on the linear scale by the quantity of one plus the ratio of the coil resistance to the shunt resistance.
In order to measure alternating current (AC), rectifiers that convert AC to DC are incorporated in the ammeter's shunt resistors. A simpler and less expensive option, with its own set of limitations, involves a moving iron (moving magnet) ammeter with its pointer attached to a counterbalance weight and a piece of soft iron; another piece of iron is located nearby. A coil surrounds the entire movement; when the current passes through the coil, magnetic fields are created around both pieces of iron which then repel each other. This results in a nonlinear movement of the pointer that corresponds to the nonlinear scale. When AC passes through the coil the magnetic fields alternate at the same frequency as the current, but the force remains one of repulsion. AC with frequencies up to 300 Hz can be measured just as well as DC. Nonetheless the moving iron ammeter has fallen into disuse, because its accuracy is limited and it has high resistance.
Modern electronics, integrated circuits, and semiconductors have ushered in a new era for measuring current. A digital display ammeter, functioning as a voltmeter (which is an integral part of the multimeter), has no moving parts, but instead indicates current by measuring the proportional voltage drop across an internal resistor. Digital displays are extremely accurate and easy to operate, but they are more expensive than their electro-mechanical cousins.
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