Page:Elementary Text-book of Physics (Anthony, 1897).djvu/203

§ 161] 160. Temperature.—Two bodies are said to be at the same temperature when, if they be brought into each other's presence, no heat is transferred from one to the other. A body is at a higher temperature than the other bodies around it when it gives up heat to them. The fact that it gives up heat may be shown by its change in volume. A body is at a lower temperature when it receives heat from surrounding bodies. It is understood, of course, in what is said above, that one body has no action upon the other, or that no work is done by one body upon the other.

161. Thermometers.—Experiments show that, in general, bodies expand, and their temperature rises progressively, with the application of heat. An instrument may be constructed which will show at any instant the volume of a body selected for the purpose. If the volume increase, we know that the temperature rises; if the volume remain constant or diminish, we know that the temperature remains stationary or falls. Such an instrument is called a thermometer.

The thermometer most in use consists of a glass bulb with a fine tube attached. The bulb and part of the tube contain mercury. In order that the thermometers of different makers may give similar readings, it is necessary to adopt two standard temperatures which can be easily and certainly reproduced. The temperatures adopted are the melting-point of ice, and the temperature of steam from boiling water, under a pressure equal to that of a column of mercury 760 millimetres high at Paris. After the instrument has been filled with mercury, it is plunged in melting ice, from which the water is allowed to drain away, and a mark is made upon the stem opposite the end of the mercury column. It is then placed in a vessel in which water is boiled, so constructed that the steam rises through a tube surrounding the thermometer, and then descends by an annular space between that tube and an outer one, and escapes at the bottom. The thermometer does not touch the water, but is entirely surrounded by steam. The point