An ammeter is a measure of the intensity of an electric current in a circuit. The unit of measure of intensity is the ampere, symbol : has.
There are several main types :
the most common analog ammeter is magneto-electrique, it uses a galvanometer to mobile framework. It measures the average value of the current that the traverse2. For measures running alternative, a bridge rectifier diode is used to straighten the power but this process allows to measure accurately currents sinusoidaux2, 3;
Ferromagnetic ammeters of table.
the ammeter ferro-magnetic (or ferromagnetic) uses two pallets of soft iron within a bobine4. One of the pallets is fixed, the other is mounted on a pivot. When current flows through the coil, the two palettes are middle and repel each other, regardless of the meaning of the courant4. This ammeter is not so polarized (it does not negative values) 4. Its accuracy and linearity are worse than those of the magneto-electrique ammeter but it can measure the effective value of current alternative form quelconque4, 3 (but frequency low < 1 kHz);
the heat meter consists of a strong thread in which to measure current flowing. This wire heats up by Joule effect, its length depending on its temperature, causes the rotation of the needle, to which it is fixed. The heat meter is not polarized. He is not influenced by nearby magnetic fields, its indications are independent of the shape (alternate or continuous form any) and the frequency of the courant5. So it can be used to measure the effective value of the alternating currents up to frequencies very elevees5. He often incorporates a temperature compensation intended to maintain its accuracy despite variations in room temperature.
Analog ammeters are increasingly replaced by digital ammeters. Yet, in practice, the observation of their needle can provide quick visual information about the variations of the measured current that the digital display gives that hard.
Fluke digital multimeter (in) 179 in ammeter position (alternative effective), caliber 10 A.
It's actually a digital Voltmeter measure the voltage produced by the current measure in resistance (called shunt). The value of the shunt depends on the caliber used. Application of Ohm's law, the voltage U measured is converted, the value of known resistance R of the shunt, into a value A corresponding to the current.
Probe (calibers 6 A, 60 A, and 300A).
There are special ammeters :
the AC current clamp is a kind of electric transformer primary is made up by the driver which we want to know the current and the secondary by winding wound on a magnetic circuit formed by the two jaws of the clamp. It is used to measure alternative currents without inserting anything into the circuit. She can not measure direct currents;
the current clamp to Hall-effect current sensor to measure currents any (alternative or continuous) and high intensity fit into the circuit without interrupting. The clamp consists of a magnetic circuit (a transformer of intensity) that closes on a semiconductor chip. This pad will be put to the induction generated by wire (common to measure) 6,7, 8. Measured induction because it has the advantage of exist regardless of the type of current. The semiconductor Pellet is subject to a perpendicular to the induction current which the traverse6, 7, 8. All of this to cause a shift of burden through the Lorentz force in the Pellet that will lead a potential difference which is proportional to the field and so aware, a feedback system requires the processor to operate at flow zero and is aware of the cancellation of the stream which, converted to voltage using a converter to operational amplifier, gives out a picture of the current mesure6 voltage;
fiber optic ammeters : they are used in the field of THT (high voltage), the mainstream and when the bandwidth of the Hall sensors is insufficient (study of violent transitional regimes, those for which the di/dt is 108 A/s above). This measurement technique using the Faraday effect : the plan of polarization of the light in the glass goes under the effect of an axial magnetic field. This effect is independent of the direction of propagation of the light but depends on the intensity;
to effect Neel ammeters are able to measure alternative, and continuous currents with high accuracy for low currents or forts10. These sensors are made up of several coils and cores made material composite nanostructure with superparamagnetiques properties, where the absence of magnetic remanence on a wide range of temperature10. An excitation coil to detect the presence of current through modulation by Neel effect. A coil of feedback to deliver the current measurement, directly proportional to the primary current and the ratio of the number of coils primary/secondaire10. The current greenhouse Neel sensor so behaves as a simple, linear transformer and precis10.
Use of an ammeter
An ammeter is connected in series in the circuit11. This means that it must open the circuit at the place where you want to measure the intensity and put the ammeter between the two terminals created by the opening of the circuit11.
Sense of connection and polarity
An ammeter measures the intensity flowing from A Terminal (or Terminal +) to Terminal COM (or Terminal-) taking into account its sign.
In general, the needle of analogue ammeters can deviate in a sens11. It needed to reflect on the direction of the current and requires wiring the ammeter to measure positive intensity : we check while terminal + of the ammeter is connected (possibly through one or more dipoles) at the pole + of the generator and that the Terminal - the ammeter is connected (possibly through one or more dipoles) to the pole - of the generateur11.
Caliber called the highest intensity that can measure the ammeter.
All modern devices are multi gauges : changing gauge by turning a switch, either by moving a card. The latest devices are autocalibrables (autorange in English) and require no manipulation.
When using an analog ammeter, avoid using a size smaller than the current. Necessary to determine by calculation an order of magnitude of this intensity and choose size accordingly. If we have no idea of the magnitude of the intensity that we are going to measure, it is desirable to leave of the highest caliber, generally sufficient. This gives an idea of the current flowing in the circuit. Then we decrease the calibre to achieve the smallest possible size, while a value greater than the measured current. However, it is necessary to proceed to the change of gauge carefully, for example cutting current or bypassing the ammeter during the change of calibre of the device, especially if the circuit is inductive.
A digital reading is direct and function of the selected size.
For the analog ammeter needle moves on a scale common to several calibers. Read indication is only a number of divisions. We must therefore infer intensity from this number taking into account the value of the gauge by making a calculation, knowing that the maximum scale corresponds to the caliber