A Staff Report from the Straight Dope Science Advisory Board

How does a touch lamp work?

September 25, 2003

Dear Straight Dope:

My mom gave me a touch lamp for Christmas, and I've been wondering how it works. Why doesn't it light when my cat touches it with her nose or paws? What makes it work? I am sure this is a silly question but I've wondered about it every since I got the thing.

Nearly all touch lamps work because of a property of the human body called capacitance, which is the ability to store electrical charge. Most touch lamps employ an oscillator, which is a type of tuned-circuit amplifier that produces an AC current, that is, one that regularly switches its polarity back and forth. The oscillator is connected to the metal parts of the lamp. When you touch the lamp's metal housing, you introduce your capacitance into the circuit. Now the oscillator has to pump charge into a much larger surface area--yours plus the lamp's. That causes the oscillator to detune, or change frequency. The lamp detects this change and toggles the light on or off. Kitty has capacitance too, but since she's much smaller, hers is a lot less than yours and isn't enough to trigger the lamp. A larger animal like an average-sized dog would, in case any touch-lamp owner is looking to teach his pet a new trick.

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