3-way lamp

Lamp bulbs with dual carbon filaments were built as early as 1902 to allow adjustable lighting levels.

When they are both energized at the same time, 150 W of power is delivered, and a high level of light is produced.

When a standard bulb is screwed into a standard socket, a matching contact on the bottom of the bulb presses against the metal tongue in the center of the socket, creating the live connection.

The metal threads of the bulb base touch the socket shell, which creates the neutral connection, and this is how the electrical circuit is completed.

A problem of these devices is that the ring contact of the socket (contact two in photo) digs into the lead seal on the lamp bulb's contact ring and this connection tends to fail early (sometimes in months), leading to intermittent flashes, popping noises, and loss of power to the low wattage filament as the lead seal on the bulb's ring alternately melts and solidifies.

Thus, if a 3-way bulb is screwed into a standard light socket that has only a center contact, only the medium-power filament operates.