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Introduction to Thermometers
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There are three different kinds of thermometers: the bulb thermometer, electric thermometer and the bimetallic strip thermometer. Let's take a look at how each one of these thermometers work.
The Bulb Thermometer
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The bulb thermometer is the simplest and one that you can make at home. It is composed of a small, thin glass tube with a rounded bulb at the bottom. This tube is usually planted on a plaque that displays temperatures in both Celsius and Fahrenheit degrees. The glass tube is filled with a liquid, which is traditionally mercury. Liquids expand when they are exposed to heat, and the bulb thermometer works on this principle. When the temperature in the area it is placed on rises, the liquid inside expands and rises in the tube. The tube is specifically calibrated to rise to a height that will correspond to the temperature it matches with on the plaque. When the temperature drops, the liquid reverses its expansion and lowers in the tube.
Bimetallic Thermomters
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Just as liquid expands as temperatures rise, so does metal. The catch here is that different metals expand at different rates. That's actually the science behind bimetallic thermometers. These are the types of thermometers found in appliances like ovens. Two strips of metal are attached to each other, and one expands faster than the other. This dual metal strip is attached inside the appliance and set just underneath a small electrode. As the temperature rises, one of the strips expands and pulls on the other as it does, which causes one end to curl up. The other remains stationary and is attached to a rivet. The end that curls will eventually come into contact with the electrode, which will stop the appliance from raising the temperature further. This is how an oven heats itself to certain degrees.
Modern Electric Thermometers
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Finally there are electric thermometers, which are most commonly found in newer appliances. They work much the same way as a bimetallic thermometer, only the circuitry inside the electronic thermometer measures the temperature through resistance. Through expansion, the device will have a higher resistance in higher temperatures, and the computer inside will make note of that. The temperature it derives is then displayed on an LCD screen.
eHow Article: How a Thermometer Works
