« PreviousContinue »
lator could again heat the inflow. As now arranged, when working above temperatures about 20° C. a small motor acts as a heart and, the tap-water being shut off, pumps the tankwater itself round through the silver tube placed above the gas-jets. The water, by passing through the pump &c., is slightly cooled thus the work of the regulator is confined to simply supplying the heat lost by convection, radiation, &c., and it performs this task admirably. As an illustration, I may mention that in the series of over 50 experiments treated of in this communication on only one occasion did the temperature of the steel chamber change by as much as Too°C. throughout the duration of an experiment. On the solitary occasion that a change amounting to nearly 6 C. was observed, the cause was found in the caking of the lime through which the gas was passed on its way to the regulator, and, in consequence, the experiment was discarded before working out its results.
A large screw, placed in the tank-water, revolved at about 800 times per minute and raised a considerable sea-t -the flow passing round, under, and over the steel chamber, the top of which was about 4 in. beneath the surface of the water.
The calorimeter was formed of brass and was suspended by glass tubes which, after passing through the lid of the steel chamber and through the water, projected above the lid of the tank. The diameter and depth were each about 10 centims. and the capacity about 700 cubic centims. Within it was suspended a silver flask with which three silver tubes communicated. One was connected with a glass tube passing to the exterior through which substances could be introduced. The second tube, which was about 18 feet in length, was, after leaving the top of the flask, twisted into a spiral within the calorimeter; the other end, terminating in the lid of the calorimeter, was then connected by a glass tube with a fourway tap entirely immersed in the outer tank. The third tube, which opened into the bottom of the flask, communicated with about 30 feet of copper tubing placed in the tank-water. The object of the whole arrangement was that the gas on leaving the calorimeter (after evaporating any liquid in the flask) should have acquired the temperature of the calorimeter, and that any gas passed into the flask should assume the temperature of the outer tank. True, this portion of the apparatus was not necessary to the experiments on aniline, but I have felt it advisable to describe it, as explaining some of the subsequent operations.
A coil of fine german-silver wire, supported on glass pillars, was placed within the calorimeter and so arranged as to Phil. Mag. S5. Vol. 39. No. 236. Jun. 1895.
surround the silver flask. Communicating with this coil
were four leads-Nos. 1 and 2 of which were fastened to one end, and Nos. 3 and 4 to the other end of the coil at the roof of the calorimeter, and, passing up through the steel vessel, communicated with the exterior. Nos. 1 and 3 formed the ends of a circuit in which were placed a high-resistance galvanometer and the Clark cells; Nos. 2 and 4 a circuit which contained a rheochord of special construction (for a description see paper J), reversing-keys, and storage-cells. It was thus possible to maintain the ends of the calorimetercoil at a difference of potential equal to that of any number of Clark cells; and I am convinced that the variations in the potential-difference during the course of an experiment rarely amounted to (The grounds on which this somewhat bold statement is based are fully given in paper J, p. 283.) The contents of the calorimeter could thus be heated or cooled without any disturbing effects from external causes. By means of the electrical arrangements above described, the supply of heat could be regulated at will and accurately determined, while, by the insertion into the flask of a volatile liquid such as ether, it was possible, by adjusting the current of dried air, to regulate the rate of cooling.
Particulars of the Clark cells have already been published (paper J, pp. 286-288), and the whole 36 maintain to-day almost exactly the same relative values as they had when compared with the Cavendish standard in 1892 there is, therefore, every reason to believe that their absolute value remains unchanged. I have not yet had time to repeat the comparison with the Cavendish standard, but I hope to do so shortly. I do not, however, anticipate that any correction will be found necessary. The whole of these cells were contained in a bath, through which the tap-water was turned by means of a regulator whenever the temperature exceeded 15° C.
The calorimeter previously described differed considerably from that used in my determinations of the mechanical equivalent. During these experiments the air-pressure in the space between the walls of the calorimeter and those of the steel chamber containing it was reduced to from 5 to 2 millim. as measured by McLeod's gauge; and my observations proved that there was a strangely rapid diminution in the loss by convection, &c., when the pressure fell below half a millimetre. I proposed, however, during the experiments for which this second calorimeter was designed to maintain the * A similar conclusion was arrived at by Bottomley; see I hil. Trans. 1887, A.
calorimeter and the steel chamber at nearly equal temperatures, and it did not, therefore, appear so necessary to guard against convection and radiation gains and losses. Previous experience had convinced me of the absolute necessity for keeping this intramural space dry, for I found that the slightest moisture in the contained air has the most astonishing effect in changing the conditions. It was therefore necessary that all joins should be absolutely tight, for the lid of the steel chamber was under water, and I proposed, in the preliminary experiments, to place water within the calorimeter. The use of indiarubber was forbidden, as I intended to insert ether in the silver flask and aniline in the calorimeter; and even if such had not been the case, indiarubber connexions are, at the best, unsatisfactory and unreliable. I decided, therefore, not to commence my experiments until I found that the apparatus was absolutely gas-tight in all parts; and I may mention that the greater part of last summer was unsuccessfully devoted to the endeavour to secure perfection in this respect. During last winter I spent considerable time in the effort to obtain some suitable medium by which to join glass to metal; and with the assistance of Mr. Thomas I was at last successful in procuring an alloy by which an air-tight join could be formed*, and which would stand considerable changes of temperature. Five glass tubes passed from the calorimeter to the steel lid, rendering ten such glassto-metal joins necessary, besides several similar ones in the exterior connexions.
In the spring of this year the intramural space was exhausted until the reading of the McLeod gauge connected therewith was reduced to 11, indicating a pressure of about 0.12 millim. The apparatus was then left untouched for a month except that the temperature was occasionally raised or lowered, and at the end of that time the reading of the gauge was still less than 12. Dry air was then readmitted to this space, and the silver flask with its connected tubes (embracing about 50 feet of tubing with several joins) tested in a similar Those who have had to deal with low pressures will understand that when all was found satisfactory a great difficulty had been surmounted. I did not retain this vacuum during the experiments, as I felt that it would subject the glass tubes &c. to a continuous strain which the conditions of the experiments rendered unnecessary. The labour had not been lost, however, for I was able to count with confidence on the gas-tightness of the whole apparatus.
I now pass to a description of that vital part of any such *Proc. Camb. Phil. Soc. 1894.
investigation, viz. the thermometry. The method I proposed to adopt necessitated extremely accurate measurements of small differences of temperature. The actual elevation was of little consequence, and therefore the use of differential thermometers immediately suggested itself. Two platinum thermometers (labelled AB and CD) were constructed with great care: four stout platinum leads passed down the stem of each, supported and insulated in the usual manner by small disks of mica, and the resistance of all these leads was made as equal as possible before attaching the coils. Great attention was given to this matter, and it is safe to assume that the leads in no case differed amongst themselves by 1 in 10,000. The coils, consisting of a particularly pure sample of platinum wire, were then attached, and several days were devoted to securing their equality. Their resistance in ice was about 18 ohms: thus 1800 of their resistance could be directly determined in the box. The galvanometer swing was about 500 for a change of 01 in the box; and such equality was secured that when both thermometers were placed in ice (the necessary precautions being taken with regard to exterior connexions &c.), no difference in the swing of the galvanometer could be observed; thus they differed by a quantity certainly less than 1 in 100,000. This equality, although not a necessity, was a great convenience.
Although cut from the same length of wire and insulated in a precisely similar manner, the coils did not possess exactly the same coefficients. The resistances in steam and sulphur were repeatedly determined and checked by observations in the vapour of aniline. Both thermometers were on several occasions heated to a red heat, the hard glass tubes containing them becoming slightly bent in the process; but since this annealing no further change has been observable in them. The method of completely standardizing such instruments has been fully described by Professor Callendar and myself in Phil. Trans. 1891, A, and I need not therefore here dwell upon it. The values of & differed slightly, viz. 1-513 and 1-511; but such a difference, even if not allowed for, would over the range 0° to 100° C. in no case cause an error exceeding about 2000 C. in elevation. These thermometers were so connected that the compensating leads of A were placed in series with the coil of B, and vice versa. Any heating of the stem of A or B, therefore, added an equal resistance to each arm of the bridge; and as the leads were everywhere bound together, the indications were absolutely 3 independent of all changes in temperature except those of the bulbs.
Great care is required to accurately determine the fixed points of such thermometers. For example, a difference of 1° at 100° C. made a difference of 067835 ohm: thus a difference of 1 millim. on the barometer caused a difference of 00244 ohm. Readings of the pressure were taken on a standardized -inch diameter mercury-barometer, and also on a compensated sulphuric barometer suspended beside it. It was found that the observations, if corrected to 100° C. by the pressure as deduced from a mercury-barometer, were not in close agreement; but if by the sulphuric-acid barometer, the results were satisfactory. On examination it appeared that the first observations of each series, when corrected for pressure, were in harmony whichever barometer was used; but such was not the case with subsequent ones of the same series. The cause is, I believe, to be found in the necessary handling of the barometer that takes place when adjusting the mercury to the ivory point. The temperature within the case undergoes a change, and the indications of the thermometer usually placed within the metal tube surrounding the barometer give but little information as to the real temperature of the mercury column, whereas the sulphuric-acid barometer requires no manipulation and is independent of such temperature changes. As an example of the consistency of the boilingpoint when corrected by the H2SO, barometer, I give the following consecutive results :
(Difference in R for 1° C. at 100°-067835.)