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Ammeter

Ammeter is a science topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Ammeter rather than just read about it. In short: An ammeter (abbreviation of ampere meter) is an instrument used to measure the current in a circuit. Electric currents are measured in amperes (A), hence the name.

Ammeter — main illustration
Ammeter — illustration

Key takeaways

  • Ammeter belongs to science; place it in that map before memorising details.
  • Learn the definition first, then one example that makes the definition concrete.
  • Connect Ammeter to a quantity you can measure, compute or draw — that is where exam questions come from.
  • Reproduce the core statement of Ammeter from memory before moving on to harder problems.

Reference excerpt

An ammeter (abbreviation of ampere meter) is an instrument used to measure the current in a circuit. Electric currents are measured in amperes (A), hence the name. For direct measurement, the ammeter is connected in series with the circuit in which the current is to be measured. An ammeter usually has low resistance so that it does not cause a significant voltage drop in the circuit being measured. Instruments used to measure smaller currents, in the milliampere or microampere range, are designated as milliammeters or microammeters. Early ammeters were laboratory instruments that relied on the Earth's magnetic field for operation. By the late 19th century, improved instruments were designed which could be mounted in any position and allowed accurate measurements in electric power systems. It is generally represented by letter 'A' in a circuit.

History

The relation between electric current, magnetic fields and physical forces was first noted by Hans Christian Ørsted in 1820, who observed a compass needle was deflected from pointing North when a current flowed in an adjacent wire. The tangent galvanometer was used to measure currents using this effect, where the restoring force returning the pointer to the zero position was provided by the Earth's magnetic field. This made these instruments usable only when aligned with the Earth's field. Sensitivity of the instrument was increased by using additional turns of wire to multiply the effect – the instruments were called "multipliers". The word rheoscope as a detector of electrical currents was coined by Sir Charles Wheatstone about 1840 but is no longer used to describe electrical instruments. The word makeup is similar to that of rheostat (also coined by Wheatstone) which was a device used to adjust the current in a circuit. Rheostat is a historical term for a variable resistance, though unlike rheoscope may still be encountered.

Types Some instruments are panel meters, meant to be mounted on some sort of control panel. Of these, the flat, horizontal or vertical type is often called an edgewise meter.

Moving-coil

The D'Arsonval 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 spiral springs to provide the restoring force. The uniform air gap between the iron core and the permanent magnet poles make the deflection of the meter linearly proportional to current. These meters have linear scales. Basic meter movements can have full-scale deflection for currents from about 25 microamperes to 10 milliamperes. Because the magnetic field is polarised, the meter needle acts in opposite directions for each direction of current. A DC ammeter is thus sensitive to which polarity it is connected in; most are marked with a positive terminal, but some have centre-zero mechanisms and can display currents in either direction. A moving coil meter indicates the average (mean) of a varying current through it, which is zero for AC. For this reason, moving-coil meters are only usable directly for DC, not AC. This type of meter movement is extremely common for both ammeters and other meters derived from them, such as voltmeters and ohmmeters.

Moving magnet Moving magnet ammeters operate on essentially the same principle as moving coil, except that the coil is mounted in the meter case, and a permanent magnet moves the needle. Moving magnet Ammeters are able to carry larger currents than moving coil instruments, often several tens of amperes, because the coil can be made of thicker wire and the current does not have to be carried by the hairsprings. Indeed, some Ammeters of this type do not have hairsprings at all, instead using a fixed permanent magnet to provide the restoring force.

Electrodynamic An electrodynamic ammeter uses an electromagnet instead of the permanent magnet of the d'Arsonval movement. This instrument can respond to both alternating and direct current and also indicates true RMS for AC. See wattmeter for an alternative use for this instrument.

Moving-iron

Moving iron ammeters use a piece of iron which moves when acted upon by the electromagnetic force of a fixed coil of wire. The moving-iron meter was invented by Austrian engineer Friedrich Drexler in 1884. This type of meter responds to both direct and alternating currents (as opposed to the moving-coil ammeter, which works on direct current only). The iron element consists of a moving 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 deflects against the restoring force provided by fine helical springs. The deflection of a moving iron meter is proportional to the square of the current. Consequently, such meters would normally have a nonlinear scale, but the iron parts are usually modified in shape to make the scale fairly linear over most of its range. Moving iron instruments indicate the RMS value of any AC waveform applied. Moving iron ammeters are commonly used to measure current in industrial frequency AC circuits.

Hot-wire

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

Digital In much the same way as the analogue ammeter formed the basis for a wide variety of derived meters, including voltmeters, the basic mechanism for a digital meter is a digital voltmeter mechanism, and other types of meter are built around this. Digital ammeter designs use a shunt resistor to produce a calibrated voltage proportional to the current flowing. This voltage is then measured by a digital voltmeter, through use of an analog-to-digital converter (ADC); the digital display is calibrated to display the current through the shunt. Such instruments are often calibrated to indicate the RMS value for a sine wave only, but many designs will indicate true RMS within limitations of the wave crest factor.

Integrating

… excerpt ends here. Continue reading the full article.

Illustrations

Ammeter: Demonstration model of a moving iron ammeter. As the current through the coil increases, the plunger is drawn further into the coil and the pointer deflects to the right.
Demonstration model of a moving iron ammeter. As the current through the coil increases, the plunger is drawn further into the coil and the pointer deflects to the right.
Ammeter: Ammeter from the University of Dundee Physics Department
Ammeter from the University of Dundee Physics Department
Ammeter: Ammeter from the old New York Penn Station terminal service plant in New York City
Ammeter from the old New York Penn Station terminal service plant in New York City
Ammeter: Wire carrying current to be measured.Spring providing restoring forceThis illustration is conceptual; in a practical meter, the iron core is stationary, and front and rear spiral springs carry current to the coil, which is supported on a rectangular bobbin. Furthermore, the poles of the permanent magnet are arcs of a circle.
Wire carrying current to be measured.Spring providing restoring forceThis illustration is conceptual; in a practical meter, the iron core is stationary, and front and rear spiral springs carry current to the coil, which is supported on a rectangular bobbin. Furthermore, the poles of the permanent magnet are arcs of a circle.
Ammeter: Face of an older moving iron ammeter with its characteristic non-linear scale. The moving iron ammeter symbol is in the lower-left corner of the meter face.
Face of an older moving iron ammeter with its characteristic non-linear scale. The moving iron ammeter symbol is in the lower-left corner of the meter face.

Worked examples

Example 1 — a first encounter with Ammeter

Start with the simplest possible case. Write down what Ammeter claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Ammeter before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Ammeter ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Ammeter

In research
Ammeter appears in science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry
Engineering practice reuses Ammeter in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom
Ammeter is common in secondary-school and first-year university syllabi. It links to neighbouring topics Ammeters, Electrical meters, Electronic test equipment, so understanding it makes those chapters shorter.
In everyday life
Look for Ammeter outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Ammeter in 20 minutes

  1. Read the reference excerpt below once, without taking notes.
  2. Close the page and write down what Ammeter means in your own words.
  3. Compare your version with the excerpt and mark what you missed.
  4. Work through the three examples above with pen and paper.
  5. Explain Ammeter out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Ammeter in simple terms?

An ammeter (abbreviation of ampere meter) is an instrument used to measure the current in a circuit. Electric currents are measured in amperes (A), hence the name.

Why does Ammeter matter?

Because it connects several science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Ammeter?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Ammeter.

Tags

  • Ammeters
  • Electrical meters
  • Electronic test equipment

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