THERMOMETER. 219 THERMOMETRY. an inclined surface by means of sharp-pointed feet at either end, creeps upward with alternat- ing rising and falling temperatures. Each step of its progress is proportional to the difl'erence between the successive maxima and minima. Thus such an arrangement will, in the course of a month, sum i) the daily ranges of temi)erature. The EvXfoHATioN Thermometer. In 1888 Dr. !Miiller-Erzbach brought out a thermometer in which a vessel full of water in the sunshine com- municates with an empty vessel in the shade. The water evaporated in the former condenses over into the latter vessel where it is absorbed by concentrated sulphVric acid. The weight lost in one vessel or gained in the other is an index to the average temperature that has prevailed in the water tlask. Comparative observations give the means of converting these weights into tem- peratures. Experimental tests of this form of integrating thermometer made at the Seewarte in Hamburg show that this apparatus gives ex- cellent results when the daily temperature varia- tions are not too great. TiiERMOGRAi'ius. This term may include all forms of apparatus in which continuous regis- tration is introduced. To this end the English Meteorological Office employs photography, main- taining, as it were, a continuous photographic register of the variations of the top of the mer- curial column. The French and the United .States Weather Bureaus use the thermographs made by Richard at Paris, in which some form of metallic thermometer is attached to a pen which moves vertically over a sheet of paper that is drawn horizontally by clockwork so that the temperature may be subsequently read otS at any given moment. Owing to the great importance of the ther- mometer in every branch of science and the arts, the principal governments of the world have es- tablished bureaus of standards for the compari- son and calibration of this and other forms of measuring apparatus. In all cases private in- dividuals may send their thermometers to these institutions for examination and may receive certificates stating the exact value of the correc- tions for their errors if any exist. Such institu- tions are the following: the Kew Observatory at London, now a branch of the Bureau of Standards; the Imperial Physical and Technical Institute at Charlottenburg. near Berlin; the International Bureau of Weights and Measures, Paris ; and the Bureau of Standards established by the United States Government at Washington, under the Treasury Department in 1900, and transferred to the Department of Commerce and Labor in 1903. See TiiERiroMETRY and the works of reference there mentioned. THERMOMETRY (from Gk. eipij.rj, tJierme, heat + -fitrpla, -inetria, measurement, from liirpov, inetron, measure). The science of as- signing proper numerical values to the tempera- ture of bodies. As explained in Heat, this in- volves several steps, the arbitrary selection of: ( 1 ) some substance as the thermometrie body, (2) some property of it which changes with the temperature, (3) two standard thermal states, e.g. melting ice and boiling water at standard pressures, (4) the scale or the number of degrees between these standard temperatures, and (5) a number to give one of these standard tempera- tures. Vol. six,— 15. By definition the 'temperature on the standard hydrogen thermometer,' Centigrade scale, is as follows: Hydrogen is the gas used, the initial pressure being 100 cm. of mercury. Change in pressure at constant volume is the etl'ect measured. The two .standard thermal states are (1) melt- ing ice and (2) vapor rising from boiling water at normal atmospheric pressure. One hundred degrees are chosen between these two temperatures. The temperature of melting ice is called 0°. Then, if ;)„ is the pressure of the gas at 0°, piM that at 100°, and p that at the temperature (<) to which a number is to be given, « = 100 ■P—Ih Pm—Po Other gases than hydrogen might be used; and the same definition would apply to temperature on the new scale as on the hydrogen scale. Slight differences, however, would alwaj's be ob- served between the numerical values for the same temperatures. If the change in volume is the heat-effect ob- served as the temperature is changed, the ob- vious definition of the numerical value of the temperature is (: 100- where t'o, loo, v are the measured values of the volume of the gas or liquid at 0', 100°. t". If the change in the electrical resistance of a given conductor is the heat-effect measured as the temperature is changed, the definition of t is R — E|,
- 100
R„ -E„ where Ro, Rim. R are the electrical resistances at 0°. 100°, f. Similarly almost any heat-effeet of any body may be taken as the one to measure in order to secure a number for temperature ; but the num- bers so found all differ in general; and if any thermometer other than the hydrogen one is actually used, the observed numbers must all be corrected by a calibration table connecting the scale of the thermometer used with the hy- drogen one. For measuring ordinary laboratory tempera- tures either mercury-in-glass or air-thermome- ters are used; for measuring extremely low tem- peratures, e.g. — 200° C, a hydrogen thermome- ter or a platinum-resistance thermometer is gen- erally used; for high temperatures, e.g. 300° C, a platinum-resistance thermometer or a thermo- couple is used. ( See Thermometek and Thermo- electricity. ) For a full discussion of these and other methods reference should be made to the Reports of the International Congress of Physics, Paris, 1900, vol. i., articles by Chappuis and Barus. The scale of temperature as defined above is known as the 'Centigrade' or 'Celsius' (q.v.) scale. There are several other scales in use. The 'Fahrenheit' scale has 32° for the melting point of ice and 212° for the boiling point of water; the 'Reaumur' (q.v.) scale has 0° for the former temperature and 80° for the latter; the 'absolute gas scale' adds to the temperature on any scale the reciprocal of the coefficient of cubical ex- pansion of hydrogen as determined on that scale — on the Centigrade scale this reciprocal
