Page:Elementary Text-book of Physics (Anthony, 1897).djvu/205

§ 162] measured in different parts of the tube. If the length vary by more than a millimetre, the tube should be rejected. If the tube be found to be suitable, a bulb is attached, mercury is introduced, and the tube sealed after the mercury has been heated to expel the air. When it is ready for graduation, the fixed points are determined; then a thread of mercury having a length equal to about ten degrees of the scale is detached from the column, and its length measured in all parts of the tube. By reference to these measurements, the tube is so graduated that the divisions represent parts of equal capacity, and are not necessarily of equal length.

If such a thermometer indicate a temperature of 10°, this means that the thermometer is in such a thermal condition that the volume of the mercury has increased from zero one tenth of its total expansion from zero to 100°. There is no reason for supposing that this represents the same proportional rise of temperature. If a thermometer be constructed in the manner described, using some liquid other than mercury, it will not in general indicate the same temperature as the mercurial thermometer, except at the two standard points. It is plain, therefore, that a given fraction of the expansion of a liquid from zero to 100° cannot be taken as representing the same fraction of the rise of temperature.

162. Air-thermometer.—If a gas be heated, and its volume kept constant, its pressurs increases. For all the so-called permanent gases—that is, those which are liquefied only with great difficulty—the amount of increase in pressure for the same increase of temperature is found to be almost exactly the same. This fact is a reason for supposing that the increase of pressure is proportional to the increase of temperature. There are theoretical reasons, as will be seen later, for the same supposition.

An instrument constructed to take advantage of this increase in pressure to measiye temperature is called an air-thermometer. A bulb so arranged that it may be placed in the medium of which the temperature is to be determined, is filled with air or some other gas, and means are provided for maintaining the volume of the gas constant, and measuring its pressure. For the reasons given above,