The circuit tester or voltage tester that uses a neon bulb is an inexpensive, extremely handy, versatile and simple-to-use electrical testing tool. The tester goes by several names, most common are circuit tester, voltage tester, test light, test lamp.
How a Voltage Tester Works
This pocket-sized tool has two probes and works with an AC line voltage of 120 volts and 240 volts. It has no power by itself but uses a neon indicator light connected to a ballast resistor, and the light only glows when power is present. As the voltage increases, the neon test light glows brighter.
The neon circuit tester is commonly used to test for a live circuit at an outlet. The tool can be purchased for under $10 from retailers such as Amazon.com and is an essential electrical testing tool for your toolbox.
This circuit tester is used to test if a circuit is energized. You connect the red test lead to the "hot" side (small slot) or black wire in an outlet and the black lead to the "neutral" side (large slot) or white wire in an outlet.
Besides checking for voltage, another way to use the tester is to check and see if an outlet or receptacle is properly grounded. To do this take one test lead and place it in the wide slot of the outlet (the neutral side). Take the other test lead and place it into the ground slot of the outlet. If the outlet is properly grounded, then the neon test bulb will not light up. If it does light up, then the outlet is wired improperly, with its polarity reversed.