A thermometer is a device used to measure how hot or cold something is. The something can be the air in a room, the human body, the water in a lake, or anything else in which the thermometer can be placed. To describe how hot or cold something is, we use the word “temperature.” On a summer day you may hear someone say, “According to my thermometer the temperature is 91 degrees.”
Thermometers can be made in many different ways. Two types are probably most familiar to you. One is the liquid filled thermometer, the kind used to take your temperature or to measure room temperature. The other is the metal thermometer with a dial and indicating needle. Your mother may use such a thermometer to measure the temperature of her oven or of a roast. Both types are based on the same principle: matter expands (takes up more space) or contracts (takes up less space) when its temperature changes.
Most liquid thermometers contain either mercury or colored alcohol. If the liquid thread inside the stem appears silvery, the liquid is mercury. If it is red, blue, or green, it is probably colored alcohol. Mercury-filled thermometers are preferred for scientific work because they tend to be more accurate. Alcohol-filled thermometers are less expensive and easier to read, the thread is more visible. In addition, alcohol does not freeze as easily as mercury. An alcohol-filled thermometer is useful in places that get very cold.
When a liquid thermometer is taken from a cold location to a warmer one, the mercury or alcohol expands. Therefore, it rises in the item. When the thermometer is moved back into the colder place, the liquid contracts. It drops to a lower position.
In the metal thermometer there is a thin strip of material wound into a spiral like a clock spring. This springlike coil is attached to the needle. When the temperature in- the coil unwinds a little, causing the needle to move toward the higher numbers. As the temperature decreases, the coil winds back, carrying the needle to the lower numbers.
To give a temperature reading, a thermometer must be equipped with a scale. Temperature is measured in degrees. And so the scale is divided into units that represent degrees. You read a liquid-filled thermometer by observing the degree mark that is level with the top of the liquid thread.
In the United States the Fahrenheit (F. ) scale is commonly used. In most other countries, the Celsius (C.) scale is favored. Scientists in the United States, too, almost always use the Celsius scale. (“Centigrade” is an older name for the Celsius scale.)
When the bulb of the thermometer is plunged into boiling water (at sea level and under normal air pressure), the top of the thread will rise until it is level with the 100 C. mark or the 212 F. mark. That is, the boiling point of pure water at normal air pressure is 100 C. or 212 F. If the thermometer is placed in melting ice under the same conditions, it will read 0 C. or 32 F. Thus, the melting point (or freezing point) of pure water is 0 C. or 32 F.