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.