Ivan Wolfson, age 13, of Philadelphia, Pa., for his question:
How does a thermometer work?
Most substances tend to expand or grow a little when heated and shrink a little when cooled. It is this trick which makes the fluid in a thermometer rise and fall to indicate temperature. This liquid cannot be colored water because water is one substance which does not follow the rule. At temperatures below four degrees Centigrade it expands, when other substances contract,,
Basically, a thermometer is a glass tube fixed onto a hollow glass reservoir. The bore, or hole through the tube, is very fine. It may vary anywhere from one‑fiftieth to one‑thousandth part of an inch. The glass reservoir and part of the bore are filled with a liquid which expands when warm and contracts when cool. The liquid is sealed in when the thermometer is made. At this time, the heat is sufficient for the liquid to swell and fill the entire bore. The level falls as it cools, leaving a vacuum,
The clinical thermometer is the type you stick under your tongue when you think you have a fever. The liquid used in most clinical thermometers is mercury, alias quicksilver. This delightful metal. is a liquid at normal temperatures. It freezes solid at minus 38.87 degrees and boils at 356.90 degrees Centigrade. This leaves plenty of clinical leeway for no human body is likely to get this cold or this hot.
The mercury thermometer is also useful to record the changing weather. Special mercury thermometers for industry are made to record. temperatures up to 500 degrees Centigrade or more. In these, nitrogen or carbon dioxide are put into the vacant part of the bore to prevent the mercury from boiling.
In regions where the temperature reaches 38 degrees below zero or lower, a mercury thermometer tends to freeze. Tinted alcohol is used. Instead alcohol thermometers are also used in certain chilly industrial processes.
The glass tube of a thermometer is marked off with a ladder of degrees. We are not satisfied to have the rising or falling liquid tell us that the temperature is rising or falling. We want to know how much and the degree marks tell us precisely what the temperature is. They may be etched on the glass tube, as on the clinical thermometer. A weather thermometer is often fixed onto a board with the degrees charted alongside the glass tube.
As the temperature rises, the fluid in the reservoir swells and pushes the line of fluid higher up in tho, glass tube. As the temperature cools, the fluid shrinks and falls in the glass tube.
The degrees may be according to the Fahrenheit or Centigrade scale. Clinical and weather thermometers use the Fahrenheit or F. scale. Thermometers used in science and industry often use the Centigrade,, or C. scale of degrees. Water freezes at 0 degrees C. and. 32 decrees F.