Mercury switch

A mercury switch is a little gadget that works like a tiny see-saw. It's made up of a little glass bulb which has some mercury inside it. The bulb also has two metal contacts, one on each end. When you tilt the switch one way, the mercury sloshes over to one end of the bulb and makes an electrical connection between the two metal contacts. When you tilt the switch back the other way, the mercury sloshes over to the other side of the bulb and makes a different electrical connection instead. People use mercury switches in all sorts of things to turn them on and off, like thermostats and security alarms. But because mercury is a really toxic substance that can hurt people and the environment, you often don't see them being used so much these days, and people are thinking of better alternatives that don't use such dangerous chemicals.