power filament for DIM, just the high-power filament for MEDIUM, and both filaments for
BRIGHT.Copyright 2001, all rights reserved
A 3-way light bulb cleverly gets by with only two filaments
Q: Why does a 3-way light bulb have only two filaments ?
A: A 3-way light bulb has two filaments: one low-power and one high-power. It combines the two to shed light at three levels.
The bulb uses just the low-power filament for DIM, just the high-power filament for MEDIUM, and both filaments for
BRIGHT.
Right: [James D. Hooker, U of Wales] A clear 3-way bulb
3-Way bulbs are always made in white finish so its nearly impossible to see the 2 filaments inside, but here's one James Hooker of Lighting Equipment News, University of Wales, UK, specially made up in a clear bulb a few years ago. You can clearly see the three wires feeding two vertical filaments, one larger than the other, at the center of the bulb.
(Answered by April Holladay, science correspondent, June 20, 2001)