A multimeter measures electrical properties such as AC or DC voltage, current, and resistance. Rather than have separate meters, a multimeter combines a voltmeter, an ammeter, and an ohmmeter. Electricians and the general public might use a multimeter on batteries, components, switches, power sources, and motors to diagnose electrical malfunctions and narrow down their cause.

The two main kinds of a multimeter are analog and digital. A digital multimeter has an LCD screen that gives a straight forward decimal read out, while an analog display moves a bar through a scale of numbers and must be interpreted. Any multimeter will work over a specific range for each measurement. Select one that's compatible with what you meter most, from low-voltage power sources to high-voltage car batteries. Multimeters are specified with a sensitivity range, so make sure you get the appropriate one.

As a voltmeter, a multimeter can measure the amount of AC or DC voltage flowing through a circuit. Voltage is a difference in potential energy between the two points. A fan, for example, should be drawing 120 volts (in the U.S.) from the plug in your wall, but a computer scanner might only draw 12 volts from a converter. To test these components, choose AC or DC, select an upper limit on the voltage, and plug the device in question right into the multimeter, without breaking the circuit. The readout should tell you whether it is functioning normally, when compared to the data specified in your user's manual.

As an ohmmeter, a multimeter finds the resistance in a circuit, which is given in ohms. You can find the resistance at any point in a circuit by first unplugging the device from a wall outlet or battery source. Then, input an approximate range you expect to contain the number of ohms. The multimeter actually passes a small amount of electricity from its own battery through the circuit to measure resistance by comparing the voltage sent out to what it receives.

When used as an ammeter, the multimeter measures current flowing through a closed circuit by interrupting that circuit. The multimeter can only be connected in series, which means that all the circuit's current will flow through the ammeter's sensors. You still need to select the range in which you expect the current to fall. This feature is used less often than the others, so some multimeters do not measure current at all.