Thermometer
an instrument for the measurement of temperature or degree of heat. It was invented by Galileo some time prior to 1611. It is founded on the principle that substances expand with heat and contract with cold, that is to say, with a loss of heat. The expanding substance used as a measure may be a liquid, as mercury or alcohol, or it may be a gas, as in the air thermometer, or even a metal.
A thermometer in common use is the mercurial thermometer or, in exceedingly cold regions, the alcohol or spirit thermometer which does not freeze up so readily. In either case the liquid is expanded by heat and is poured into a glass tube with a large bulb at its lower end. While the liquid is still hot the tube is sealed to prevent the entrance of air. The colder the temperature, the less the bulk of the mercury or alcohol, and the lower the column stands in the tube. With increasing heat the liquid swells, and the column rises higher.
Several systems of marking the thermometer are in use. According to the Fahrenheit system, the temperature of chipped ice and salt is marked 0 deg. F.; that of boiling water is 212 deg. F.; the intervening space is divided into 212 degrees. Water freezes at 32 deg. F. At one time, the temperature of ice and salt was supposed to be that of the greatest attainable degree of cold. Still lower temperatures are said to be below zero. Thus,--32 deg. F. is 64 degrees colder than freezing water at 32 deg. F.
According to the centigrade system the zero point may be found by placing the thermometer in freezing water. A lower temperature is marked below zero. The point at which the mercury stands when placed in boiling water is marked 100 degree C. The intervening distance is divided into 100 degrees. Thus we say that 0 degree C. and 100 degree C. correspond to 32 degree F. and 212 degree F. A degree Fahrenheit is 1/180 of the difference between the freezing and the boiling temperatures for water. A degree centigrade is 1/100 of the same difference. A degree centigrade is therefore equal to 180/100 or 9/5 of a degree Fahrenheit.
Reaumur's thermometer, once used largely in Germany and Russia but now giving way to the centigrade, agrees with the latter in calling the freezing point of water zero, but differs in calling the boiling point 80 degree. A Reaumur degree, therefore, is equal to 100/80 or 5/4 of a centigrade degree, and to 180/80 or 9/4 of a Fahrenheit degree.
John Hopkins university has a thermometer said to be valued at $10,000. The gradation is so fine that a microscope is required to read the temperature.
By reason of its convenient notation and natural zero point, the centigrade thermometer is gaining in popularity and will no doubt become universal.