It's morning. You stumble downstairs, put the coffee-pot on, and throw some bread into the toaster. You push the button and a minute or two later, bingo! Up pop two perfectly browned pieces of toast, ready for a bit of butter and jam.
But how does the toaster control the toast's temperature? We all know how to operate a toaster: simply place the slices of bread onto the spring loaded rack and push the lever. The bread descends into the toaster, and heating elements that brown all sides of the bread are switched on.
As the temperature inside the toaster rises and the toast browns, the heat sensor, a metal strip located on the inside of the toasting cage, expands and bends outward. When the temperature of the toast is high enough, the strip touches another strip of metal known as the trip plate, completing an electric circuit.
Once the electric circuit is completed, an electric current begins to run through a cylindrical coil of wire, or solenoid. The electric current creates an electromagnetic field around the solenoid. The magnetized solenoid attracts the catch, a small piece of metal which, in turn, is attached to a lever that releases the toast rack.
You can control the toast's darkness by adjusting the browning control. When you adjust the dial for lighter toast, the trip plate moves toward the heat sensor, lessening the distance between them, and causing the toast rack to be released sooner. But if the toaster is already hot, how does slice after slice of bread emerge golden brown and crunchy? The heat sensor is next to the slice of bread in the toasting cage, so it moves when the toast , not the toaster, is hot enough.
You can see the toaster's mechanism for yourself if you want. Make sure your toaster is unplugged and cool, then look inside it on the "one slice only" side and you'll see the heat sensor, which is a small piece of wire. This mechanism also explains why the "one slice only" message is on the toaster--it's there to tell you to put the bread on the side where the sensor is so it can tell when the toast is done.