Kent Wada, age 11, of Vancouver, B.C. Canada, for his question:
How can a thermometer show temperature?
This clever gadget can do its work because heat is a form of energy and tiny atoms use this particular energy to speed up and spread apart. This is why substances tend to expand as they get warmer. Some substances expand more readily than others. These may be used to make thermometers and a scale of degrees is added to measure temperature from expansion.
The average thermometer is a glass tube fixed onto a hollow glass ball. The fine hole through the tube is called the bore and its width is between one fiftieth and one thousandth part of an inch. The glass ball holds a reservoir of mercury and surplus mercury reaches part way up the tube. We can observe these facts merely by looking at an ordinary thermometer.
When the little gadget is manufactured, things are quite hot and enough mercury is sealed inside the glass to fill both the round reservoir and the tube. As things cool down, the level of mercury in the tube falls which leaves a small vacuum at the top of the tube. Now the sealed mercury is all set to take temperatures, though we need something extra to translate its information.
But first let's see why mercury is selected for this particular work. At ordinary temperatures, this fascinating metal is a liquid. It boils at 673.84 degrees F. and refuses to freeze solid at temperatures above minus 37.95 degrees F. Certainly the temperature of the human body never goes to these extremes. The weather never reaches the boiling point of mercury, though in polar regions it has been known to drop below mercury's freezing point.
Between these two points, slight changes in the temperature cause liquid mercury to expand and contract. This is because its atoms use heat energy to move faster and spread apart. So, as the temperature rises, the amount of sealed in mercury increases. The only place it can go is up the tube. Hence, as the temperature rises the thread of mercury rises higher in the glass tube.
This sensitive metallic liquid also responds to slight heat losses. In this case, the loss of heat causes its atoms to slow down and move closer together. As the sealed in mercury loses heat, it contracts and slides down the tube.
All this is very interesting information. But it does not tell a thing about how a thermometer takes your temperature. For this job we need a measured scale of degrees. This shows precisely how much the mercury expands and contracts. And from this we know just how much heat was gained or lost to cause the changes.
A weather thermometer and the average fever thermometer are measured in Fahrenheit degrees. On this scale, the temperature of boiling water is 212 degrees and its freezing point is 32 degrees. Most scientists use the Centigrade or Celsius scale. When we change to the metric system, everybody will use the simpler C scale on which water freezes at 0 degrees and boils at 100 degrees.