130 PYROMETER of a nitrogen or hydrogen gas-thermometer, whether it is arranged to show the increase of pressure at constant volume or the increase of volume at constant pressure, give for the temperature numerical results which are practically identical with the corresponding numbers on the absolute scale. It follows, therefore, that any two gas -thermo- meters, if similarly graduated, would give identical indica- tions for the same temperature, no matter whether or not they are filled with the same kind of gas and whether or not the quantities of the gases are such that the pressure in the two thermometers is the same at any one temperature. This important property of gas -thermometers has been experimentally verified by Regnault l by direct comparison up to 350 C. of instruments filled with different gases and at different pressures. For these reasons the readings of a properly arranged gas-thermometer have justly come to be regarded as furnishing the standard of temperature, at any rate outside the limits of the freezing and boiling points, and indeed may now be regarded as the tempera- ture standard for scientific purposes throughout the whole range. The Kew standards are calibrated mercury-in-glass thermometers whose fixed points are repeatedly redeter- mined. Such instruments will not agree exactly with the gas-thermometer except at the freezing-point and boiling- point. Comparisons have been made between various mercury-thermometers and air-thermometers by Regnault 2 and many others. The results obtained by different ob- servers are not entirely concordant ; but it is needless here to discuss them, for, whatever may be the divergence between the mercury and air thermometers in the freezing- point and boiling-point, the method of measuring higher temperatures by continuing the scale of a mercury-thermo- meter beyond those limits is altogether untrustworthy in consequence of the very wide divergence between different mercury-thermometers at the same temperature, amount- ing sometimes to 10 or more 3 at a temperature of 300. The air-thermometer readings must therefore be regarded as the standard at any rate for temperatures beyond the boiling-point. The general principle employed in the use of the gas- thermometer is as follows. Let p Q be the pressure of a mass of gas at C., p loo the pressure of the same mass of gas at 100 C., the volume being the same, p t the observed pressure of the same mass of gas at some unknown tem- perature t, the volume still remaining the same, then /IN Pw-Po 100" We require, therefore, three observations of the pressure, two 4 to graduate the instrument and the third to measure the temperature. If the thermometer has been filled with gas of a perfectly definite kind e.g., properly dried and purified air, nitrogen, or hydrogen and the containing vessel has been previously thoroughly dried, the value of
- > 100 may be obtained from tables, since p m =p (l + lOOa),
where a is the tabulated coefficient of expansion of the gas at constant volume. It is practically impossible to keep the volume of the gas constant in consequence of the expansion of the envelope. A correction must be applied on this account, the value of which is derived from inde- pendent observations of the expansion of the material of the envelope. If the pressure of the gas be maintained constant, and the volumes v t , v 100 , v be observed for the three temperatures t, 100, 0, we have 1 1 "De la Mesure des Temperatures," Mem. de VInst., xxi. p. 168. 2 Mem. de VInst., xxi. p. 191. 3 See HEAT, sec. 26. 4 The two known temperatures at which the pressure is measured need not necessarily be and 100, though these are often the most convenient. The formula requires only slight modification to make it applicable when any two other known temperatures are adopted. In like manner v m - v may be taken from a table of the coefficients of expansion of gases. The different methods which have been suggested for the employment of this property of gases to measure high temperatures are very numerous. We give details of a few of them. (1) The Constant - Pressure Method. The following is a very simple and practical plan of employing the method for obtaining a reading of the temperature. A glass or porcelain bulb, provided with a fine neck, is very carefully dried and filled with perfectly dry air ; it is then exposed to the source of the heat whose temper- ature is to be investigated in such a manner that the point of the neck just projects from the furnace. When the equilibrium of temperature is reached, the neck is hermetically sealed by a blow- pipe or oxy-hydrogen flame, and the bulb is withdrawn and allowed to cool, and weighed. The neck is then immersed in water or mercury and the point broken off. In consequence of the previous expansion of the air the pressure in the interior is much less than the atmospheric pressure, and the liquid consequently enters the bulb. When so much has entered that the pressure is the same inside and out (the difficulty of the comparative opacity of the porcelain is not insurmountable), the end is closed by a small piece of wax, and the bulb removed and weighed, with the liquid it contains. The bulb is then completely filled with the liquid, and weighed a third time. The difference between the third and first weighings gives a value v t of formula (2), which only requires correction for the expansion of the envelope, while the difference between the second and first weighings gives a value of the volume from which V Q and v m can be calculated, using the known co-efficient of expansion of air, and thus all the requisite data for the deter- mination of t are obtained. This method was used by Regnault 5 to determine the coefficient of expansion of air, and has since been described as "a new pyrometer." In the process just described the volume of the residual gas is measured ; its pressure, after cooling, may be measured instead, by an arrangement which was suggested by Regnault. The bulb is provided with a long fine neck, to the end of which a tap is fitted and so arranged that it can be easily connected with a manometer. The bulb is exposed to the high temperature, the tap being left open, and when the final temperature is reached the tap is closed and the bulb allowed to cool ; it is then connected with the manometer, and, if the tap be a three- way tap, drilled as shown in fig. 1, it is easy to expel all the air from the bulb side of the manometer, between the mercury surface and the tap. The residual pressure is then measured by the manometer. A correction is required for the expansion of Fig. 1. the bulb and for the part of the connecting tube not exposed to the high temperature. Instead of measuring the volume of the residual gas in the manner thus described, Deville and Troost 6 have pumped the hot air out of the porcelain bulb by means of a Sprcngel pump, and measured the volume of air delivered by the pump. On this plan a series of observations can be made at the same temperature, a three-way tube with suitable taps serving to put the bulb alter- nately in connexion with a vessel to supply dry air and with the pump. Crafts and Meier 7 have obtained results by sweeping out the air with a current of hydrochloric-acid gas, which was separated from the air it carried by being passed through water. An instrument for observing the continuous variation of volume of a gas at constant pressure is figured and described by Sir W. Thomson in HEAT (sec. 65). Arrangements have also been sug- gested by which the density 8 of the gas at the high temperature can be directly measured. Regnault* has described a hydrogen pyrometer based on this principle suitable for measuring the temperature of a porcelain furnace. A wrought-iron tube of known capacity is permanently fixed in the furnace ; it is filled with pure dry hydrogen by passing a current of the gas through it for some time. The current of gas is then stopped, and after the gas has attained the temperature of the furnace it is swept out by a current of dry air and passed over red-hot copper oxide. The water thus 5 Mem. de VInst., xxi. 6 Comples Rendus, xc. 727, 773. 7 C. R., xc. 606. 8 Throughout this article the term " density " is used whenever the mass of a unit of volume of a substance is referred to. Ann. de Chim., [3], Ixiii. p. 39.
