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An ammeter is a device used to directly measure electric current in amperes. The electric current represents a directed movement of free electrons under the influence of an electric field. Ampere is a unit for measuring the current intensity. Ampere is one of the 7 main variables in the SI system. Its name comes from the French physicist Andre-Marie Ampere who discovered the electromagnetism. The scales of ammeters are marked in microamperes, milliamperes, amperes or kiloamperes, depending on the operating range of measuring of the device.
As a rule, the ammeter is switched sequentially in the circuit, as opposed to the voltmeter.
The internal resistance of the ammeter has to be infinitesimal. Ideally the internal resistance equals zero. Any value of the resistance of the device, which is other than zero, will affect the measured current because after connecting the ammeter in the electric circuit, besides the load resistance, the measurements will be affected by the resistance of the ammeter.

The greater the difference between the load resistance and the internal resistance of the ammeter, the more accurate is the device under equal conditions.
Very often, when the practice requires increasing the measuring range of the ammeter, a shunt is used.
– in circuits with AC and DC, transformer of current (usually with transmitting ratio 100/5 or 250/5, 75/5)

– only in circuits with AC and magnetic amplifier for circuits with DC only.

Ammeters for measuring only DC are magneto-electric, for measuring AC – detection and induction ammeters, for both alternating and direct current – electromagnetic, electro-dynamic, thermoelectric and heat ammeters. They are produced as portable devices (accuracy 0.1-0.5) or for control panels (1-2.5). Ammeters, like voltmeters, are used not only for measuring specific electrical magnitude at a particular time, but also for the systematic monitoring of current and voltage in a monitored electrical installation. For this reason, these devices have certain protections and alarms which get activated if the current or voltage exceeds a predetermined critical point.

Shunt resistors, connected in the ammeter in parallel, are used for measuring large currents. Most of the current passes through the shunt circuit and only a small portion passes through the ammeter, which allows the ammeter to measure larger currents. As shunting components have very little resistance, if we connect the ammeter in parallel with a source of voltage, there will be short circuit, which can switch out the fuse and /or damage the device. Ammeters, as well as voltmeters, are often integrated in a complex multifunctional device, such as a multi-meter. The first multi-meters were even called avometers (from ampere-voltmeters).
The most common ammeters are those in which the moving part of the apparatus represents a pointer, fixed motionless on an electromagnet, which is rotated about its axis in proportion to the current flowing through its coil, and as a result the pointer moves along the scale. A spring is placed to counteract the torque. When the force of the spring equals the force with which the current passing through the coil rotates the pointer, it will show the magnitude of the current which has passed through the ammeter. This simple device could be provided with an electronic circuit which amplifies the current which flows through the coil. Very often there is added protection which saves the ammeter from burning as a result of improper connection, etc. In recent years, the number of devices with liquid crystal display or LEDs has been increasing.

In electro-dynamic systems the balance of the system is achieved in terms of two coils, connected in series or in parallel (both circuits find application), one of which is movable and the other is stationary. The pointer is attached to the movable coil. The current, which flows through the coils, causes deflection of the pointer on the scale.

The scales on ammeters show the degree of accuracy of the devices, information whether the ammeter is used for direct or alternating current (or both), as well  as the measuring units (A, mA, kA, etc.). Sometimes the measuring system is shown schematically, for example the electromagnetic system is described by a simply presented magnet. There are also ammeters for DIN rails with an angular scale, which are placed in electrical panels, where authorized persons control the parameters of the electrical installation.

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