You have probably used an ammeter in a physics lab to measure the current in an electric circuit. Besides being useful for teaching purposes and understanding the flow of electrons, ammeters are actually a vital part of many electrical systems around us. Once a circuit, much more complicated than that constructed in a high school physics class, is built, it's important to check its functionality. Some examples would include the electricity in buildings, engines in automobiles, and the power supply of a computer. If the current flowing through a particular system exceeds its limits, it can result in a malfunction and even become dangerous. That's where the ammeter is useful. In this article, we'll discuss the various theoretical and practical aspects of ammeters!
Ammeter Definition
Measuring electric current is a crucial aspect of evaluating the performance of various electronics and power systems. We can do that by utilizing an ammeter visible in Figure 1 below.
Fig. 1 - A typical ammeter with two ranges for measurements.
An ammeter is a tool used to measure the current at a specific point within a circuit.
It's easy to remember, as the name stems directly from the measurement of current - amperes. It must always be connected in series with the element in which the current is measured, as that's when the current stays constant.
An ideal ammeter has zero resistance, meaning it doesn't affect the current in the element it's in series with. In reality, that's obviously not the case: all ammeters have at least some internal resistance, but it has to be as low as possible, as any resistance present will alter the current measurements. An example problem comparing the two cases can be found later in this article.
An equivalent tool to measure the electric potential difference between two points in a circuit is a voltmeter. By connecting a voltmeter before and after a consumer (e.g. a resistor) we can measure the voltage drop.
Ammeter Symbol
Just like every other component in an electric circuit, ammeters have their own symbol. It's easily recognizable, as the letter "A" confined within a circle, pictured in Figure 2 below, stands for the ammeter.
Fig. 2 - The ammeter symbol.
Sometimes, the letter may have a wavy line or a straight line paired with a dotted line above it. This simply indicates whether the current is AC (alternating current) or DC (direct current), respectively.
Ammeter Formula and Functions
The main formula to consider when dealing with ammeters is Ohm's law:
\[I=\frac{V}{R},\]
where \(I\) is the current in amperes (\(\mathrm{A}\)), \(V\) is the voltage in volts (\(\mathrm{V}\)), and \(R\) is the resistance in ohms (\(\Omega\)). If we measure the current using an ammeter and the voltage using a voltmeter, we can then calculate the resistance at a certain point in a circuit.
Similarly, if we know the resistance and voltage of the circuit, we can double-check our ammeter's measurements. It's important to apply the correct equation for calculating the resistance of the circuit. An ammeter is always going to be connected in series, while a voltmeter has to be connected in parallel. Recall that:
If the resistors are in series (i.e., next to each other), you add the value of each resistor together: \[R_\mathrm{series}=\sum_{n}R_n=R_1+R_2+ \cdots,\]
If the resistors are in parallel, the rule for finding the total resistance is as follows: \[\frac{1}{R_\mathrm{parallel}}=\sum_{n}\frac{1}{R_n} =\frac{1}{R_1}+\frac{1}{R_2}+\cdots.\]
Let's apply these equations to an example problem, comparing the current in a circuit with an ideal ammeter versus a non-ideal one!
A series circuit has two resistors, \(1\,\Omega\) and \(2\,\Omega\) respectively, and a \(12\,\mathrm{V}\) battery. What is the measured current of this circuit if it has an ideal ammeter connected to it? How does this current change if a non-ideal ammeter with an internal resistance of \(3\,\Omega\) is connected instead?
Fig. 3 - An electric circuit diagram with an ammeter connected in series.
Answer:
First, let's consider the ideal ammeter cases. As the name implies, in this case, the ammeter has no resistance, so we use the following equation to find the total resistance of this series circuit:
\begin{align} R_\mathrm{series}&=R_1+R_2 \\ &= 1\,\Omega + 2\,\Omega\\ &=3\,\Omega. \end{align}
We can use Ohm's law
\[I=\frac{V}{R}\]
to calculate the current that the ammeter should be detecting:
\[I=\frac{12\,\mathrm{V}}{3\,\Omega}=4\,\mathrm{A}.\]
Now, let's follow the same steps, only this time accounting for the internal resistance of the ammeter:
\begin{align} R_\mathrm{series}&=R_1+R_2+ R_\mathrm{A}\\ &= 1\,\Omega + 2\,\Omega+3\,\Omega\\ &=6\,\Omega. \end{align}
Therefore, the current measured by the non-ideal ammeter is
\[I=\frac{12\,\mathrm{V}}{6\,\Omega}=2\,\mathrm{A}\]
which is two times smaller than that of an ideal ammeter.
Based on these results, we can conclude that the internal resistance of the ammeter can have a significant impact on the measurement of the actual current flowing through the circuit.
Ammeter Function
The main function of an ammeter is to measure the current in an electric circuit. So, let's walk through the basic steps of applying an ammeter to a circuit in real life. An example diagram of a typical ammeter is visible in Figure 4 below. It has a scale displaying a range of currents that it will be able to detect and a positive and a negative connector indicated on its base. Sometimes, there are two scales overlaying each other, each of which will have a separate positive connector. These usually consist of a broader and narrow range of measurements, for example, \(-1\) to \(3\), and \(-0.2\) to \(0.6\) pictured in Figure 1, allowing us to take more accurate measurements within this smaller range.
Fig. 4 - An ammeter diagram.
In a simple circuit consisting of a battery, source (e.g., a lightbulb), and wires, we can measure the current by disconnecting the wire from the source and the battery and inserting the ammeter inside the circuit.
The negative connector of the ammeter should be connected to the negative terminal of the battery. Similarly, the positive connector connects to the positive terminal. All that's left is to read the measurement of the current and estimate the error!
Effect of Temperature
Due to the sensitivity of an ammeter, whenever taking measurements, we should be cautious about the surrounding temperatures. Fluctuations in temperature can lead to false readings. For instance, if the temperature increases, so do the resistance. Greater resistance means less current will flow through it; therefore the ammeter reading will be lower as well. This effect can be reduced by connecting swamping resistance to the ammeter in series.
Swamping resistance is a resistance with a zero temperature coefficient.
Ammeter Measures
This article focuses on ammeters in particular. However, nowadays, there are other instruments used to measure the current of an electric system.
For instance, a common instrument used to measure current is a multimeter.
A multimeter is a tool that measures electric current, voltage, and resistance over several ranges of value.
Fig. 5 - A multimeter encompasses the functions of an ammeter, voltmeter, and ohmmeter.
As the definition implies, it's a very versatile tool that can provide us with a lot of information about a particular circuit. Instead of having to bring an ammeter, voltmeter, and ohmmeter, it's all combined in a singular instrument.
Another similar instrument to an ammeter is a galvanometer.
A galvanometer is a tool used to measure small electric currents.
The main difference between the two tools is that the ammeter measures only the magnitude of the current, while the galvanometer can also determine the direction. However, it works only for a small range of values.
Conversion of a Galvanometer into an Ammeter
It is possible to convert a galvanometer into an ammeter by simply adding a shunt resistance \(S\) to the circuit. It has a very low resistance and must be connected to the galvanometer in parallel, as pictured in Figure 6.
Fig. 6 - A shunt resistance connected in parallel to a galvanometer.
We know that the potential resistance across two parallel components is the same. So by applying Ohm's law, we conclude that the current \(I\) is directly proportional to the current flowing through the galvanometer \(I_\mathrm{G}\) based on the following expression:
\[I_\mathrm{G}=\frac{S}{S + R_\mathrm{G}}I\]
where \(R_\mathrm{G}\) is the resistance of the galvanometer.
If we want to increase the range of a galvanometer, we apply
\[S=\frac{G}{n-1},\]
where \(S\) is the shunt resistance, \(G\) is the resistance of the galvanometer, and \(n\) is the number of times the resistance increases.
Ammeter - Key takeaways
- An ammeter is a tool used to measure the current at a specific point within a circuit.
- An ammeter must always be connected in series with the element in which the current is measured, as that's when the current stays constant.
- An ideal ammeter has zero resistance, meaning it doesn't affect the current in the element it's in series with.
- The symbol for an ammeter in an electric circuit is the letter "A" confined within a circle.
- The main formula to consider when dealing with ammeters is Ohm's law \(I=\frac{V}{R}\).
- A multimeter is a tool that measures electric current, voltage, and resistance over several ranges of value.
References
- Fig. 1 - Ammeter (https://commons.wikimedia.org/wiki/File:%D0%90%D0%BC%D0%BF%D0%B5%D1%80%D0%BC%D0%B5%D1%82%D1%80_2.jpg) by Желуденко Павло is licensed by CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/).
- Fig. 2 - Ammeter symbol, StudySmarter Originals.
- Fig. 3 - Ammeter connected in a series circuit, StudySmarter Originals.
- Fig. 4 - An ammeter diagram, StudySmarter Originals.
- Fig. 5 - A DMM on the desk (https://unsplash.com/photos/g8Pr-LbVbjU) by Nekhil R (https://unsplash.com/@dark_matter_09) on Unsplash is licensed by Public Domain.
- Fig. 6 - Shunt resistance connected parallel to a galvanometer, StudySmarter Originals.
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