Ammeter, which one to choose and why?

November 2, 2021

An ammeter is a measuring instrument used to measure current in a circuit. Electric currents are measured in amps (A), which is where it gets its name. Instruments used to measure smaller currents, in the milliamp or microampere range, are referred to as milliammeters or microammeters.

The first ammeters were laboratory tools that depended on the earth’s magnetic field for their operation. At the end of the 19th century, some improved tools were designed that could be mounted in any position and allowed precise measurements in electrical power systems. It is usually represented by the letter ‘A’ in a circle.

Next, we will talk in detail about one of the most used laboratory instruments, such as the breathalyzer , in history. If you want to know more, join us until the end.

What is an ammeter?

The instrument used to measure current is called an ammeter. Current is the constant flow of electrons whose unit of measurement is the ampere. But practically the ammeter has a small internal resistance. The measuring range of the ammeter depends on the resistance value.

There are two types of electrical current: direct current (DC) and alternating current (AC). DC sends current in one direction, while AC alternates the direction of the current at regular intervals.

History of the ammeter

The tangent galvanometer was used to measure currents using this effect, where the restoring force that returned the pointer to zero was provided by the Earth’s magnetic field.

The relationship between electric current, magnetic fields, and physical forces was first noticed by Hans Christian Ørsted . Who, in 1820, observed that a compass needle deviated from pointing north when a current flowed in an adjacent wire.

This made these instruments usable only when they were aligned with the earth’s field. The sensitivity of the instrument was increased by using additional turns of wire to multiply the effect: the instruments were called “multipliers.”

The word for electric current detector was used by Sir Charles Wheatstone around the 1840s. But this is no longer used to describe electrical instruments. The word is similar to rheostat, also coined by Wheatstone, which was a device used to adjust current in a circuit. Rheostat is a historical term for a variable resistor, although unlike the rheoscope it can still be found.

Ammeter function

Ammeters work to measure electrical current through current through a set of coils with very low resistance and inductive reactance . This allows for a very low impedance, the force that opposes electrical current, which allows the ammeter to accurately measure current in a circuit without interference or change due to the ammeter itself.

In the moving coil ammeter, it works with the resulting movement of the fixed magnets configured to oppose the current. The movement then turns a centrally located armor that is attached to an indicator dial. This dial is set on a graduated scale that allows the operator to know how much current is moving through a closed circuit.

You must connect an ammeter in series when you measure current in a circuit. The low impedance of the ammeters means that you won’t lose a lot of power. If the ammeter were wired in parallel, the path could be shorted so that all current flows through the ammeter instead of the circuit.

Ammeter principle

The main principle of the ammeter is that it must have a very low resistance and also an inductive reactance. Now why do we need this? Can’t we connect an ammeter in parallel? The answer to this question is that it has a very low impedance because it must have a very low voltage drop and it must be connected in series connection because the current is the same in the series circuit.

Also, due to the very low impedance, the power loss will be low, and if connected in parallel, it becomes a near-shorted path. And, all current will flow through the ammeter as a result of the high current that the instrument can burn. For this reason, it must be connected in series. For an ideal ammeter, it should have zero impedance so that it has zero voltage drop so that the power loss across the instrument is zero.

Effect of temperature on the ammeter

The ammeter is a sensitive device that is easily affected by ambient temperature. The temperature variation causes the error in the reading. This can be reduced by the resistance of the swamp. The resistance that has a zero temperature coefficient is known as the swamp resistance. It is connected in series with the ammeter. Swamp resistance reduces the effect of temperature on the meter.

The ammeter has a built-in fuse that protects the ammeter from strong current. If a substantial current flows through the ammeter, the fuse will blow. The ammeter cannot measure current until the new one replaces the fuse.

Types of ammeter

Mobile bovine ammeter

The galvanometer is a moving coil ammeter . It uses magnetic deflection, where current passing through a coil placed in the magnetic field of a permanent magnet causes the coil to move. The modern form of this instrument was developed by Edward Weston, and uses two coil springs to provide the restoring force.

The uniform air gap between the iron core and the permanent magnet poles makes the meter deflection linearly proportional to the current. These meters have linear scales. The basic movements of the meter can have a large-scale deviation for currents from about 25 microamps to 10 milliamps.

Moving Magnet Ammeter

The ammeters moving magnet function essentially the same principle that the coil, except that the coil is mounted in the housing of the meter and a permanent magnet moves the needle. The moving magnet ammeter can carry larger currents than moving coil instruments, often several tens of amps. This is because the coil can be made of a thicker wire and the current does not have to be carried by the coils.

Electrodynamic ammeter

An electrodynamic ammeter uses an electromagnet instead of the permanent motion magnet. This instrument can respond to both AC and DC current and also indicates true RMS for AC. See Wattmeter for an alternative use for this instrument.

Iron ammeter

The ammeters iron in motion using a piece of iron which moves when acted upon by the electromagnetic force of a coil wire fixed. The moving iron meter was invented by the Austrian engineer Friedrich Drexler in 1884. This type of meter responds to both direct and alternating currents. Unlike the moving coil ammeter, which operates on direct current only.

The iron element consists of a movable vane attached to a pointer and a fixed vane, surrounded by a coil. As alternating or direct current flows through the coil and induces a magnetic field in both vanes, the vanes repel each other and the moving vane is deflected against the restoring force provided by fine coil springs

Cable ammeter

In a hot wire ammeter , a current passes through a wire that expands as it heats up. Although these instruments have a slow response time and low precision, they were sometimes used to measure radio frequency current. These also measure true RMS for an applied AC.

Digital ammeter

In the same way that the analog ammeter formed the basis for a wide variety of derived meters, including voltmeters, the basic mechanism for a digital meter is a digital voltmeter. And other types of meters are built around this. Digital ammeter designs use a shunt resistor to produce a calibrated voltage proportional to the current flow. This voltage is measured with a digital voltmeter, using an analog-to-digital converter.


There is also a range of devices known as integrating ammeters . In these ammeters, the current adds up over time, resulting in the product of current and time. Which is proportional to the electrical charge transferred with that current.

Ammeter vs galvanometers

The galvanometers detect the strength and direction of the currents in sensitive circuits. A pointer connected to the coil moves on a scale. The scale is calibrated to read the current in amps. Galvanometers require a magnetic field, while ammeters can work without one.

While a galvanometer is much more accurate than an ammeter, it is not as accurate. This means that galvanometers can be very sensitive to small changes in current, but this current could still be far from the true value.

Dr. Loony Davis5
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Born and raised in Brussels in an English family, I have always lived in a multicultural environment. After several work experiences in marketing and communication, I came to Smart Water Magazine, which I describe as the most exciting challenge of my career.
I am a person with great restlessness and curiosity to learn, discover what I do not know, as well as reinvent myself daily, someone who is curious about life and wants to know. I enjoy sharing knowledge.
This is my personal project but I also collaborate in other blogs, it is the case, the most important web on water currently exists in the US, if you are interested you can read my articles here.

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