ON A N E W FORM O F CONSTANT VOLUME AIR T H E R MOMETER, W H I C H SHOWS THE T O T A L PRESSURE DIRECTLY, AND MAY B E GRADUATED I N DEGREES BY TEMPERATURE'
BY J. R . ERSKINE MURRAY
5 I . The thermometer described in the present communication is a constant volume one. Its advantages proceed mainly from a simple arrangement whereby tlie total pressure of the enclosed air, and hence its temperature, since these are' proportional, is measured directly by the height of one column of mercury. T h e external atmospheric pressure is elimB inated by the adjustment of an auxiliary reservoir of mercury. Q 2. I n Fig. I , A is the airbulb, B is the pressure-gauge, Absolute which is an ordinary barometer tube with a vacuum above the mercury connected at its lower end to the bent stem of the airbulb and also by a flexible tube to the reservoir C. D is the constant volunie mark on the stem of the air-bulb. T h e part of the instrument which is filled with mercury when the air is at oo C is shaded in the diagram. 3. To make an observation FIG.I of temperature the mercury is adjusted to the mark D by raising or lowering the reservoir. T h e -
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'From tlie Proc. Roy. SOC. Erlinb.
21,299 ( aS97).
height of the column in B above the level of the mark D now gives the total pressure of the enclosed air. T h e air-bulb and pressure gauge may be cut off entirely from external pressure by closing the stop-cock between the pressure-gauge aiid the reservoir. If the absolute temperature corresponditig to any given value of the pressure be known, we can calculate that corresponding to any other pressure by simple proportion. Thus, in order to graduate the thermometer in degrees of temperature, it is only necessary to find the total pressure for any one temperature ; that of melting ice, which we may take as 273' on the absolute scale of temperature, is the iiiost convenient. T o fix this point on the scale, the bulb is iinmersed in melting ice and the mercury adjusted to the volume mark by raising or lowering the reservoir. The height, h, of the mercury column in B above the level of the mark now corresponds to 273" absolute. But, since pressure and temperature are in simple proportion, the degrees at all parts of the scale are of equal length ; thus 121273 is the length of one degree. Q 4. I t will be seen that the reservoir is merely a device for supplying mercury at the proper pressure to fill up the gauge-tube, and that it may be replaced by an)' suitable mechanical device, such as a cylinder full of mercury in which a piston is forced down by a screw. It is obvious that the height of the mercury in B above its level in the open reservoir C always corresponds to the atmospheric pressure at the moment. This portion of the apparatus is in fact a barometer if the reservoir C be open, but it is not in the least essential that it should be so. Q 5 . T h e atmospheric pressure is, after all, merely an ncn'dent; and its constant recurrence i n all calculations on the pressure of gases is very misleading to the student, who is apt to look upon it as a factor of a different nature to the observed pressure in the ordinary air thermometer, and hinders his realization of the fact of the simple proportionality of temperature and pressure or volume, S 6. The length of a degree on this thermometer depends only on the total pressure at some given temperature ; hence it is clear that the sensibility of the thermometer may be altered to suit any purpose by choosing a proper pressure at which to fill the bulb with air at the standard temperature. Thus, if great sensibility be
7'6
J . R. E. Murray
required, the air when immersed in melting ice should be under great pressure, making the height h , which is equivalent to 273") large ; while, if a great range of temperature be required without great sensibility, it is only necessary m to partially exhaust the bulb, at the same time placing the reservoir below the volume mark, so that the atmospheric pressure may be partially counterbalanced. T h e length of the pressuregauge tube of the therinometer may thus be indefinitely reduced, though of course at low pressures the open reservoir would require to be placed at nearly the barometric height below the volume mark. T h e easiest way to obtain at 0°C. at a pressure somewhat below that of the atmosphere is to fill the bulb with air at a high temperature, balancing the reduction of pressure and maintaining the volume constant as it cools by lowering the reservoir. When at 0°C the air will now have a pressure below that of the atmosphere. Q 7. In order to facilitate the correction for capillarity, on account of difference in the diameters of FIG.2 the pressure-gauge tube and the volume-gauge, the constant volume inark may be put in at the end of the fine tube at the stem, just where it joins the wider tube which forms its downward continuation. At this point the tube is conical, and its walls make an angle of about 45 degrees with the horizontal. Hence the mercury surface when at this point will be a plane, and its surface tension will act entirely in a horizontal direction.
New Form of Coilstant Vohme Air Thermometer
717
8. In order to be able to adjust the quantity of air in the bulb while filling it, a small capillary, E, is drawn out from the side of the stem at a point near the volunie mark. This allows the air to escape as the mercury is introduced. The capillary is sealed before the reading of the pressure at o°C is taken. S 9. I t would be possible to give the thermometer a greater apparent simplicity by omitting the auxiliary reservoir and making the presswe-tube movable, as in Joly’s thermometer. T h e siinplification is more apparent than real, however, and introduces several disadvantages. I t would be impossible to graduate the pressuretube itself, and hence some other arrangement would have to be used. T h e clamp which supported the main tube would also have to be in a more inconvenient position than that for the reservoir in the form in which I have described (see fig. 2).
