peratures, multiply by and add 32; to change from F. to C. temperatures subtract 32 and multiply by §. 168. Range of the mercury thermometer. Since mercury freeezes at-39° C., temperatures lower than this are very often measured by means of alcohol thermometers, for the freezing point of alcohol is -130° C. Similarly, since the boiling point of mercury is about 357° C., mercury thermometers can not be used for measuring very high temperatures. For both very high and very low temperatures-in fact, for all temperatures a gas ther mometer is the standard instrument. 169. The standard hydrogen thermometer. The modern gas thermometer (Fig. 152) is, however, widely different from that devised by Galileo (Fig. 148). It is not usually the increase in the volume of a gas kept under constant pressure which is taken as the measure of temperature change, but rather the increase in pressure which the molecules of a confined gas exert against the walls of a vessel the volume of which is kept constant. The essential features of the method of calibration and use of the standard hydrogen thermometer at the International Bureau of Weights and Measures at Paris are as follows: FIG. 152. The standard gas thermometer First the bulb B (Fig. 152) is filled with hydrogen and the space above the mercury in the tube a is made as nearly a perfect vacuum as possible. B is then surrounded with melting ice (as in Fig. 149) and the tube a is raised or lowered until the mercury in the arm b stands exactly opposite the fixed mark c on the tube. Now, since the space above D is a vacuum, the pressure exerted by the hydrogen in B against the mercury surface at c just supports the mercury column ED. The point D is marked on a strip of metal behind the tube a. The bulb B is then placed in a steam bath like that shown in Fig. 150. The increased pressure of the gas in B at once begins to force the mercury down at c and up at D. But by raising the arm a the mercury in b is forced back again to c, the increased pressure of the gas on the surface of the mercury at c being balanced by the increased height of the mercury column supported, which is now EF instead of ED. When the gas in B is thoroughly heated to the temperature of the steam, the arm a is very carefully adjusted so that the mercury in b stands very exactly at c, its original level. A second mark is then placed on the metal strip exactly opposite the new level of the mercury; that is, at F. Then D is marked 0° C., and F is marked 100° C. The vertical distance between these marks is divided into 100 exactly equal parts. Divisions of exactly the same length are carried above the 100° mark and below the 0° mark. One degree of change in temperature is then defined as any change in temperature which will cause the pressure of the gas in B to change by the amount represented by the distance between any two of these divisions. This distance is found to be of the height ED. 3 In other words, one degree of change in temperature on the centigrade scale is such a change in temperature as will cause the pressure exerted by a confined volume of hydrogen to change by of its pressure at the temperature of melting ice (0°C.). 1 273 170. Absolute zero and absolute temperature. Since cooling the hydrogen through 1o C., as defined above, reduces the pressures of its value at 0° C., it is clear that cooling it 273° below 0° C. would reduce its pressure to zero. But from the standpoint of the kinetic theory this would be the temperature at which all motions of the hydrogen molecules would cease. This temperature is called the absolute zero, and the temperature measured from this zero is called absolute temperature. Thus, if A is used to denote the absolute scale, we have 0° C. 273° A., 100° C. = 373° A., 15° C. = 288° A., and so on. It is customary to indicate temperatures y on the centigrade scale by t and on the absolute scale by T. We have, then, = Fig. 153 gives a comparison of the absolute scale with the Fahrenheit and centigrade scales. The mercury thermometer yields temperatures which differ from the absolute scale by an amount that is negligible for ordinary purposes (less than 2° between 0° and 100°). F C(t) A(T) 212+ 100+ 373 +Boiling 32° 0273+Freezing 171. Low temperatures. The absolute zero of temperature can, of course, never be attained, but in recent years rapid strides have been made toward it. Fifty years ago the lowest temperature which had ever been measured was 110° C., the temperature attained by Faraday in 1845 by causing a mixture of ether and solid carbon dioxide to evaporate in a vacuum. But in 1880 air was first liquefied, and was found, by means of a gas thermometer, to have a temperature of 180° C. When liquid air evaporates into a space which is kept exhausted by means of an air pump, its temperature falls to about 220° C. Fahrenheit, centigrade, and Recently hydrogen has been liquefied and has been found to have a -459.4-2731 Absolute zero (No heat) FIG. 153. Comparison of absolute scales temperature at atmospheric pressure of - 243° C. All these temperatures have been measured by means of hydrogen thermometers. By allowing liquid hydrogen to evaporate into a space kept exhausted by an air pump, Dewar in 1900 attained a temperature of 260°. In 1923 Kamerlingh Onnes, using liquefied helium, attained a temperature of - 272.18° C., or less than 1° C. above absolute zero. SUMMARY. Centigrade temperatures are changed to Fahrenheit by multiplying by and adding 32. Fahrenheit temperatures are changed to centigrade by subtracting 32 and multiplying by. 5 One of the best known and most prolific of nineteenth-century physicists; born in Belfast, Ireland; professor of physics in Glasgow University, Scotland, for more than fifty years; especially renowned for his investigations in heat and electricity; originator of the absolute thermodynamic scale of temperature; formulator of the second law of thermodynamics; inventor of the electrometer, the mirror galvanometer, and many other important electrical devices This shows the relative sizes of Fulton's Clermont, the first successful steamboat, and the Leviathan, the largest ship flying |