A circuit tester is a device which is used to test a circuit to determine whether or not power is reaching the circuit. Circuit testers are very inexpensive tools which can be obtained at hardware and home suppliers, and they are critical tools for anyone who works with electricity to have. While they do not provide the detailed information available with a multimeter, they are useful for quick checking of electrical circuits, and they take no time at all to learn to use.
Typically, a circuit tester has a light which illuminates when a circuit is getting power. Some may generate buzzing sounds, and the volume of the sound or the intensity of the light may increase with the amount of power available to the circuit. This can be useful for differentiating between circuits which are being supplied with different amounts of power, or for identifying circuits which are getting too much or too little power, which can be a sign of an electrical short or a similar problem.
To use a circuit tester, one of the probes is placed on the hot or positive pole of the circuit, and the other is attached to either the neutral or the ground. With something like a power outlet, this can be done by sticking the probes into the two primary holes of the outlet. Circuit testers can also be used to test exposed circuits, such as that found under the hood of a car, and they can be used to see whether or not power is reaching junction boxes, wiring for chandeliers, and a variety of other electrical situations.
While using a circuit tester, it is important to only handle the insulated portions of the device. Handling the probes directly could expose someone to a shock or create a short circuit. For people who want more information about the circuit, it will be necessary to use a multimeter to gather data. Multimeters are also very useful tools to have for people who work with electrical circuits and electrical components, although they tend to be more expensive and finicky than basic circuit testers. It's also possible to build one's own circuit tester, using two insulated wires which attach to probes on one end and a lightbulb on the other.
By getting into the habit of using a circuit tester before working on any circuit, people can greatly reduce the risk of injuries or property damage caused by unknowingly working with an energized circuit. Even professional electricians utilize circuit testers, because electricity can be very dangerous, and people can very easily make mistakes which can be identified and addressed with a circuit tester.