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Pete C.

wrote:

I have a bedside lamp with a 100W halogen lamp which runs directly off
110V. In the base of the lamp is a small rotary switch about 3/4" in
diameter. It has three positions: Full intensity, reduced intensity
and off. No other parts are discernible.

The question is: How does the lamp achieve the reduction in light
intensity (about half)?

I actually know the answer because I took the switch apart but when I
was looking for it before on Google etc. I could not find it.

I am posting here to see how widely this trick is known.

A diode. Cut out half of the AC cycle and get reduced brightness. Very,
very common, indeed there are/were little socket insert disks made that
you stuck in a light socket to save power.