Thermometer
Collier's New Encyclopedia (1921)
in physics, an instrument for measuring intensity of heat or temperature, by means of expansion of a liquid or gas. Mercury is generally employed, and an or ordinary thermometer consists of a spherical or cylindrical glass bulb at the end of a very fine tube, the bulb being completely filled and the tube partly filled, with mercury, while the space above the mercury contains only a small quantity of mercury vapor, which offers no resistance to the expansion of the mercury. A rise of temperature is indicated by a rise of the mercury in the tube, owing to expansion and, conversely, a fall of temperature is indicated by a fall of the mercury in the tube. A graduated scale is attached with two fixed points: the lower, or freezing point, and the upper, or boiling point, of water. The distance between the two fixed points is then divided into a certain number of equal parts, or degrees, which are continued above and below the two fixed points. On the Centigrade or Celsius thermometer (used by scientific men all over the world and in general use on the continent of Europe), the distance between the two points is divided into 100 degrees, the freezing point being 0°, and the boiling point being 100°; on the Réaumur thermometer (used only in N. W. Europe), the distance is divided into 80 degrees, the freezing point being 0° and the boiling point 80°; on the Fahrenheit thermometer (in general use in the W United States and England), the distance is divided into 180 degrees, but, since zero is 32 degrees below the freezing point, the freezing point is 32°, and the boiling point is 212. Degrees above 0° are termed + degrees, while those below 0° are termed degrees. C.5×9+ 32 = F. F.-329 X 4 = R. 5X4R. 49+ 32 = F. C. R. 4×5= C. F.-329 × 5 = C. R. Mercury can only be used for temperatures between - 40° and +675°, since it freezes at 40° and boils at 675°. For lower temperatures alcohol is used; and for high temperatures air thermometers are employed, in which changes of temperature are measured by the expansion or contraction of a known volume of air. In deep-sea thermometers, used for ascertaining the temperature of the sea, the bulb is specially protected against the pressure of the water.
What Has Changed in the Last Century
Collier's in 1921 treated mercury-in-glass as the normal instrument; today mercury thermometers have been largely driven off the market, with the 2013 Minamata Convention on Mercury committing signatories to phase out mercury measuring devices and US states and the EU having already banned mercury fever thermometers. Most everyday measurement is now electronic — thermistors and platinum resistance sensors, thermocouples for furnace heat, and infrared devices that read a temperature from the radiation of a forehead or a distant surface without touching it. The Réaumur scale the entry lists as "used only in N. W. Europe" is now a museum curiosity, and Fahrenheit survives in general use essentially only in the United States. Temperature itself is no longer defined by the ice and steam points at all: since 2019 the kelvin, and with it the Celsius degree, has been fixed by an exact value of the Boltzmann constant.
Written for this edition, 2026.