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5. On a Constant Pressure Gas Thermometer
Published online by Cambridge University Press: 15 September 2014
Extract
In the article on “Heat” published in the eleventh volume of the Encyclopœdia Britannica, referred to in my previous communications to the Royal Society on Steam Pressure Thermometers, it is shown that the Constant Pressure Air Thermometer is the proper form of expansional thermometer to give temperature on the absolute thermodynamic scale, with no other data as to physical properties of the fluid than the thermal effect which it experiences in being forced through a porous plug, as in the experiment of Joule and myself on this subject; and the thermal capacity of the fluid under constant pressure. These data for air, hydrogen, and nitrogen have all been obtained with considerable accuracy, and thererfore it becomes an important object towards promoting accurate thermometry, to make a practical working thermometer directly adapted to show temperature on the absolute thermodynamic scale through the whole range of temperature, from the lowest attainable by any means, to the highest for which glass remains solid.
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- Proceedings 1879–80
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- Copyright © Royal Society of Edinburgh 1880
References
page 539 note * “Thermal Effects of Fluids in Motion,” Trans. Roy. Soc. Lond., June 1853, June 1854, June 1860, and June 1862.
page 540 note * Common air is inadmissible, because even at ordinary temperatures its oxygen attacks mercury. The film of oxide thus formed would be very inconvenient at the surface of the mercury caulking, round the base of the piston, and on the inner surface of the glass tube to which it would adhere. Besides, sooner or later the whole quantity of oxygen in the air must be diminished to a sensible degree by the loss of the part of it which combines with the mercury. So far as we know, Regnault did not complain of this evil in his use of common air in his normal air thermometer nor in his experiments on the expansion of air (“Expériences,” vol. i.), though probably it has vitiated his results to some sensible degree. But he found it to produce such great irregularities when, instead of common air, he experimented on pure oxygen, that from the results he could draw no conclusion as to the expansion of this gas (“Expériences,” vol. i. p. 77). Another reason for the avoidance of air or other gas containing free oxygen is to save the oil or other liquid which is interposed between it and the mercury of the manometer from being thickened or otherwise altered by oxidation.
page 541 note * This screw is to be so well fitted in the iron sole-plate as to be sufficiently mercury-tight without the aid of any soft material, under such moderate pressure as the greatest it will experience when the pressure chosen for the thermometric gas is not more than a few centimetres above the external atmospheric pressure. When the same plan of apparatus is used for investigation of the expansion of gasses under high pressures, a greased leather washer may be used on the upper side of the screw-hole in the sole-plate, to prevent mercury from escaping round the screw. It is to be remarked that in no case will a little oozing out of the mercury round the screw while it is being turned introduce any error at all into the thermometric result; because the correctness of the measurement of the volume of the gas depends simply on the mercury being brought up into contact with the bottom of the piston, and not more than just perceptibly up between the piston and volumetric tube surrounding it.