The ammeter, also known as an ammeter , is an instrument used to measure the intensity of the electric current present in an electrical circuit ; this measuring instrument is directly calibrated in amperes or its decimal fractions (the ampere , A, is – in the International System – the unit of measurement of the intensity of the electric current).

To carry out the measurement, an electromechanical system, which varies according to the type of instrument, causes the rotation of an index with respect to a calibrated scale so that the deviation of the index from the zero position corresponds to the value of the intensity of the current expressed in amperes .

Definition of ammeter

Ammeter is the measuring instrument used to measure the current in the circuit. It measures the small amount of current in milliamps or microamps. The ammeter is placed in series with the measuring circuit so that all the current in the circuit passes through it.

The resistance of the ammeter is very small compared to the voltmeter. For an ideal ammeter, the resistance value is equal to zero. The small resistance does not hinder the flow of current and therefore the ammeter measures the actual value.

Classification of ammeters

Ammeters can be divided into two groups based on the type of current they can measure ( direct current or alternating current ).

However, ammeters can also be grouped – as explained below – based on the sensitive element of the instrument; also in this case it is possible to reduce these measuring instruments to two fundamental types:

  • the one, based on magnetic effects, measures the intensity of the electric current by exploiting the mutual actions between the field created by it and a reference field created by a permanent magnet or another fixed coil;
  • the other, based on thermal effects, measures the intensity of the current by exploiting the dimensional variations originating from the heating of a wire carrying an electric current ( Joule effect ).

Digital ammeter

Electronic instruments can display the measured quantity with an index that moves on a graduated scale, but more often they are now static and display the measurement with a number on the display. A digital ammeter is a digital voltmeter placed in parallel with a high precision shunt resistor inside the instrument. There are various types of digital voltmeters composed of integrated circuits with complex functions. But basically they compare the analog voltage to be measured with an internally generated sample voltage, doing a sort of counting that digitizes the voltage to be measured and whose result is translated into the number that appears on the display.
Therefore, among the functional blocks that constitute them, the fundamental one is the analog/digital converter which implements the transformation of the analog voltage into digital quantity. But the conversion requires other sophisticated electronic circuits. There are single-ramp, simple-integration, double-ramp and multi-ramp voltmeters. Good quality digital voltmeters allow you to measure voltages from 10 nV to 1000 V with a measurement speed of 1 ms. Among the many advantages they have compared to analogue instruments, it is worth remembering that they can interface with a personal computer.

Analog ammeters

There are numerous analog ammeters. We mention the most used over time, many of which have gone down in history and are now exhibited in museums. In all of them, the shunt resistor must be insensitive to temperature variations; generally, a manganine plate resistor is used.

Analog ammeters

Magnetoelectric ammeter :

It is a rectangular moving coil ammeter immersed in the field of a permanent magnet. Derived from the Deprez-d’Arsonval galvanometer and modified by Weston who replaced the supporting conductor wires with revolving pins with semi-precious stone supports, taking inspiration from the oscillating balance of watches, and using spiral springs for the torque antagonistic to the torque generated by the coil. It only measures direct current, but with a rectifier it can measure the effective value of alternating current and in this case two scales and special connections are normally required. This type was highly appreciated for its sensitivity and precision with linear scale.

Magnetoelectric ammeter :
Magnetoelectric ammeter :

Moving iron ammeter and fixed coil :

The device of the instrument is made up of a fixed copper coil carrying current and, inside it, the moving unit made up of a small suitably shaped soft iron core, eccentric with respect to the axis on to which the index is fixed, in the case of attraction, or by two cylindrical segments of soft iron (one fixed and one mobile connected to the axis with the index) in the case of repulsion. The opposing torque is obtained either with a counterweight or more often with a spiral spring. The fixed coil can be of a couple of turns for capacities of a hundred amperes or of many turns for capacities of the order of tens of amperes with the direct insertion instrument. The flow rates can be different with the use of a shunt placed in parallel to the ammeter.

Electrodynamic ammeter :

Derived from the electrodynamometers of the late 19th century, it is essentially made up of a moving coil immersed within the field of a simple or double fixed coil wound in air. If these are wound on ferromagnetic cores, the instrument is called a ferrodynamic ammeter, which is characterized by stronger driving torques, but is less precise. The fixed and moving coils can be connected in series for milliammeters. If the moving coil is connected in parallel to the fixed ones (placed in series with each other) higher flow rates are obtained. Then there are construction details due to good functioning requirements which would require much more detailed explanations. It was the best instrument for AC measurements and also suitable for DC measurements. In the first photo you can see an electrodynamic ammeter from Siemens & Halske; in the second photo of an electrodynamic crew from Allocchio Bacchini you can clearly see the two coils: fixed and mobile.

Induction ammeter (for alternating currents only) :

The simplest of these instruments consists of a thin aluminum or copper disc which can rotate between the poles of two electromagnets generally placed at 90° to each other as in the figure taken from L Olivieri E. Ravelli, Elettrotecnica Misure Elettriche Vol. The two alternating currents that flow through the two electromagnets are out of phase with each other by an angle on which the intensity of the driving torque depends. It is maximum for the 90° angle. The explanation of how this type of ammeter works is quite long, therefore it goes beyond this context. It must also be added that there are three widely used variants: induction ammeter with a single electromagnet and shaded conductor pole, which covers half the surface of the pole; induction ammeter with a single electromagnet of particular construction with a short-circuited coil placed on a part of the polar core; rotating field induction ammeter, with a lamellar drum of a particular shape and with the two electromagnets also arranged and connected in a very particular way.


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