DIE AIR OR GAS THERMOMETER. This form of the instrument is accepted as the standard and the indications of mercurial, alcohol, or other thermometers are all supposed to be corrected so as to agree with the standard adopted by the International reau of NVeights and Aleasurea at Paris. This standard consists of a hollow bulb of platinum filled with pure, dry nitrogen. When kept at a uniform pressure the volume of the gas expands in direct proportion to its tem perature. By calling the volume at the melting point of ice V, and the volume at the boiling point of water V + 100°. we establish a scale of centigrade degrees, which may be extended above or below these limits, so that, for instance, the temperature of the absolute cold or entire absence of heat would be —273° C. and the temperature of the boiling point of mercury would he about +357° C. Numerous precautions are needed in the use of the air ther mometer, all of which are explained in the Travaux et memoires of the In ternational Bureau of Weights and Measures, and in Guillaume, Ther mometric (Paris, 1890).
According to the theories of thermodynamics the peculiarities of platinum, nitrogen, or of any other materials that may be used, introduce small ir regularities, so that the scale of equal changes of volume is not exactly proportional to the quan tity of heat or the change in temperature. There fore some recommend that a slight correction be ap plied to the nitrogen gas thermometer in order to obtain the ideal thermo dynamic equivalent. In Fig. I we see the simplest or portable form of air thermometer. as arranged by Joly, for measuring temperatures below the boiling point of water.
The large glass bulb on the left is filled with dry air, or dry nitrogen, and ends in a short vertical glass tube attached to the flexible rubber tube whose other end opens into another short glass tube, shown on the right-hand side, whose upper end is hermetically closed and constitutes the vacuum chamber of a barometric column of mercury. The glass bulb is immersed in a liquid or gas whose tem perature is to be measured. if the volume of the inclosed gas expands with the higher tem perature that is to be measured, then this ex pansion is counterbalanced by raising the vacuum chamber and the flexible tube on the right until the increased pressure of the mer cury column compresses the expanding gas in the large bulb so as to keep it at a constant volume, which is indicated by the fact that the mercury in the left-hand tube just touches the tip of a fine glass point that is permanently fastened inside the tube. The pressure exerted by the
mercurial column is measured by the difference in the level of the mercury in the right-hand and left-hand tubes. The scale for this meas urement is graduated on a strip of mirror set into the vertical framework that supports the bulb and the tubes.
In Fig. 2 we see an arrangement of the air thermometer for more exact measurements, etc. The thermometric substance (nitrogen) is in closed in the platinum bulb A, which is sup ported inside of a bath, B, which is so ar ranged that the hot gas or liquid whose tem perature is to be measured may completely sur round the bulb by flowing in at E and out, at F. D D are metal screens. There are a number of mercurial thermometers, T, which are at the same temperature as the bulb. The bulb commu nicates with the barometer at G by the tube C, the connection being shown enlarged at V. The pressure of the mercurial column needed to keep the gas in the bulb at a constant volume is that due to the height of the column from X to Y, and may amount to several atmospheres, or to a height of many feet. The lower end of the mer curial column, or the tube H, 1, .T, K. is fixed, but the upper tube. P Q, can be raised indefi nitely. As Q rises inside of the large tube. J, more mercury may be needed, and this is sup plied from the reservoir, 11, through the flexible tube. L. The difference in height between the mercurial surfaces at X and Z is measured by a reading telescope or eathetometer. M is the upright standard carrying the apparatus, N a movable cradle for the barometer tube worked by the adjusting screw 0. S is a similar ar rangement for the reservoir, R, with its adjust ing screw, U.