Click on, Click off!

May 28, 2009 7:39 AM

Most lamp switches require two clicks to turn on or off... why do they need 4 positions for only two states (on and off)?

Every lamp switch (of the turning variety) that I've run into, you have to twist it two clicks to turn it on, and two clicks to turn it off... is there a design reason for this, is it electrically necessary?

posted by frwagon to home & garden (8 answers total)

Yes, there is a design reason: three-way bulbs.
posted by hariya at 7:40 AM on May 28, 2009

Some light bulbs have three different levels of brightness, so three positions for dim, medium, and bright and one for off.
posted by procrastination at 7:41 AM on May 28, 2009

Then why are two of the positions off and two on? I would think that if the settings are "dim, regular, bright, and off" then three of the positions would be on if you were using a standard bulb.
posted by soelo at 7:51 AM on May 28, 2009

http://ask.metafilter.com/93557/Can-a-oneway-CFL-bulb-be-used-in-a-threeway-socket#1368862
posted by hariya at 7:54 AM on May 28, 2009

oops, that was to answer your off off, on on question.
posted by hariya at 7:55 AM on May 28, 2009

Then why are two of the positions off and two on? I would think that if the settings are "dim, regular, bright, and off" then three of the positions would be on if you were using a standard bulb.

A normal bulb has a single filament. A three-way bulb has two filaments, one brighter than the other. So the dim setting turns on the dimmer filament, the medium the brighter one, and bright turns on both. The off setting turns off both. So each of the two filaments is on in two of the positions, and off in the other two.

When you put a regular bulb in, the single filament gets turned on in place of one of the pair of filaments in the three-way bulb, probably the brighter one. So since the brighter one is off when the lamp is off or in the dim setting but on in the medium and brightest settings, it appears that there are two off positions and two on ones when there is a regular bulb in there.
posted by procrastination at 8:04 AM on May 28, 2009 [4 favorites]

Hey cool! I've been underutilizing several of my lamps without knowing it.
posted by orange swan at 8:07 AM on May 28, 2009

If you take the bulb out and look inside the socket, you'll see a contact in the center, and another contact which is a ring around the center.
posted by Chocolate Pickle at 10:35 AM on May 28, 2009