Screw bases for 3-way light bulbs have three contact wires soldered on the light bulb base. The wire soldered to the side of the light bulb remains the same as the standard screw base light bulb. However, whereas the standard screw base has one contact wire connected to the bottom of the base, the 3-way light bulb base has two contact wires soldered to the bottom of the light bulb in two different locations.

The solder at the bottom of a 3-way light bulb base is divided into two voltage contact points by a plastic ring. At the bottom of the 3-way light bulb base, there is a small circle of solder, a plastic ring surrounding the solder, and another ring of solder around the plastic ring. One contact wire is connected by the inner solder circle, and the other contact wire is connected by the outer solder ring.

When the light bulb is switched off, no contact is made to any solder point, and the light bulb is not lit. When the light bulb is switched to its first setting, voltage makes contact on the side soldered contact wire and the inner top solder circle. The electrical current flowing through these two points excites the filament or ballast to generate its lowest light output. When the light bulb is switched to the second setting, voltage makes contact on the side solder point and the outer solder ring on the bottom of the base. The electrical current flowing through these two points excites the filament or ballast to generate its medium level of light output. Finally, when the light bulb is switched to the third setting, voltage makes contact on all three solder points. The electrical current flowing through all three contact wires excites the filament or ballast to generate its highest light output. Note that the side contact wire will always make contact with the voltage in each setting of a 3-way light bulb.

If a standard screw base light bulb is placed inside a 3-way fixture, the light bulb will only turn on with its full light output. Only light bulbs with 3-way bases can operate properly in 3-way fixtures with low, medium, and high light output levels.

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