Voltmeter

Voltmeter function and symbol

So, how exactly do voltmeters work, and what can we use them for? All voltmeters must be placed in parallel with the component they are trying to measure the voltage of. This is because components in parallel have the same potential difference, as per Kirchhoff's Loop Rule, and so by placing the voltmeter in parallel, it measures the voltage across the component by measuring its own voltage.

Different voltmeters do this in different ways. Early voltmeters measured the current flowing through them by using electromagnetic inductance to deflect a needle which, when combined with a resistor of known resistance, allowed the voltage to be calculated via Ohm's Law. Modern digital voltmeters tend to measure the discharge time of a capacitor to calculate the voltage across them.

Fig. 1 - An analog voltmeter uses electromagnetic induction to measure the potential difference.

As can be seen in figure 2, the symbol of a voltmeter in the circuit diagram is a circle with the letter V in the center.

Fig. 2 - The symbol for a voltmeter within circuit diagrams.

Voltmeters can measure the potential difference across any two points of a circuit, and so maybe in parallel with multiple components if the total potential difference of the components is being measured. For example, the voltmeter \(V_1\) measures the voltage of the bulb, but the voltmeter \(V_2\) measures the voltage of both the bulb and the resistor.

Fig. 3 - It's important to check which components are in parallel with the voltmeter as multiple components can be measured by the same voltmeter as seen above.

Voltmeters are designed to measure the voltage across with as little impact on the current through the component as possible. To ensure this, voltmeters have very high resistances to stop current flowing through them. When considering ideal voltmeters within circuit diagrams, they are assumed to have infinite resistance and so can measure the voltage without any current flowing through them. This is not possible for real voltmeters, and so voltmeters will always affect a circuit somewhat.

Voltmeter unit

Voltage is defined as energy per unit charge and can be calculated using the equation\[\text{Voltage}=\frac{\text{Work Done}}{\text{Charge}}.\]

As energy is measured in joules \(\mathrm{J}\) and charge in coulomb \(\mathrm{C},\) it follows that the unit for voltage is joules per coulomb \(\frac{\mathrm{J}}{\mathrm{C}}\). This defines the standard unit of voltage, the volt \(\mathrm{V}=\frac{\mathrm{J}}{\mathrm{C}}\) named after the Italian physicist Alessandro Volta.

The voltage of a battery or power source determines how much energy it gives to a current, and so is an indicator of how powerful it is. In the USA, the mains electricity supply, used for household appliances like microwaves, runs at \(120\,\mathrm{V}\) whilst in much of the rest of the world, the mains are at \(240\,\mathrm{V}\). The sort of battery voltages you will see in circuit analysis, for example, during lab experiments, will likely be around \(5 \, \mathrm{to} \, 10\,\mathrm{V}\).

Voltmeter diagram

Voltmeters appear in many circuit analysis questions, so it's important that we can recognize voltmeters in a diagram as well as know how to construct diagrams with voltmeters in them. The key fact to remember is that voltmeters must be drawn parallel with whatever component or components they are measuring.

Let's take a look at an example circuit question involving voltmeters.

Voltmeter vs. Multimeter

You may be familiar with the instrument shown in Figure 5, from using them in lab work. These are multimeters and are often used to measure the voltage of a component, much like voltmeters. In fact, multimeters are simply instruments that can be used to measure multiple different electrical quantities, such as current, voltage, and resistance.

Fig. 5 - Multimeters are vital for electrical engineers, allowing different quantities to be measured with the same instrument.

Multimeters are essentially just an extension of single-use voltmeters and ammeters, which measure current. They contain multiple settings so that different quantities can be measured. However, placing multimeters in parallel with components is still necessary if we want to measure their voltage. A multimeter placed in series will not give a reading if on the voltage setting, though it would give a reading if on the current setting, as it would then act like an ammeter.