Thermometer

(-mom'e-tur.) [From Gr. therme, heat, and metron, a measure.) (Phys.) Although literally a measure of heat, the instrument known as the T. -the invention of which is generally attributed to Galileo-does not measure absolute quantities of heat; it serves to indicate variations of sensible heat in two or more bodies-that is, to show whether one substance contains more or le sensible heat than another, and the relationships between such differences. The ordinary T. consists of a slender gla tube, having a bulb at one end, and being hermetically sealed at the other. The bulb and a portion of this tube contains mercury, or alcohol colored-almost always the former; the rest of the tube is a vacuum. When the temperature is raised or lowered, the fluid expands or contracts, so as to occupy a portion of the tube le or greater than before; and the amount of expansion and contraction is indicated by a scale attached to the instrument. There are two fixed points in a T., from which the graduation is made: one the freezing-point of water, which is unchangeable; the other the boiling-point of the same fluid, which also, with a given atmospheric pre ure, is invariable. The zero, or 0, of Fahrenheit is 32 °below the freezing-point; it is the temperature obtained by a mixture of snow and salt, which he erroneously supposed was the lowest po ible; the boiling point of water is 212°. The T. of Fahrenheit is chiefly used in this country and in England. In the Centigrade T., the zero is the freezing-point; and 100° the boiling-point of water. This T. has been adopted in France, and very generally in science. In the T. of Reaumur, the zero is also the freezing-point of water; but 80° is the boiling-point of water. This T. is used in [s. 892]