Silent Switch
Manufactured prior to 1991 by many manufacturers, mercury light switches (generally referred to as “silent switches” contain about 2 grams of mercury. Mainly found in older buildings, these devices look like typical wall switches, but they do not make the audible "click" sound when activated. The mercury switch has a small hollow cylinder, partially filled with mercury; when in the “off” position, a small contact is above the mercury level, when flipped to the “on” position, the contact is immersed in mercury. This establishes contact between the two terminals and provides power to the light fixture.
