OF THE NATIONAL PHYSICAL LABORATORY. 347 



IT]. The. Scale, <Cr. 



The manometer scale, nearly two metres long, is graduated into millimetres and 

 carries a slider with vernier and fine adjustment screw by which the position of the 

 mercury column in B could be read directly to '02 millim. Over the part of the 

 scale used in these experiments no corrections were required. 



Accurate setting of the mercury to the point in the closed limb of the manometer 

 was facilitated by the use of a small short-focus telescope clamped to the stand. 

 Measurements of the height of this point on the scale, as given by the vernier 

 readings when the sighting edge of the slider was brought in line with the point, 

 were taken frequently throughout the work. The transfer of level was effected by a 

 Quincke microscope fixed on a plate-glass table which could be attached to the gas 

 thermometer supports. This constant was different in the several sets owing to the 

 tube A having for different reasons been several times rebuilt. 



The Rejwrvoirs and their Attachment, &c. 



For some early experiments up to 200 ('. reservoirs of normal Jena glass If/" were 

 fixed having a long capillary stem attached by fusion. For the later series at high 

 temperatures reservoirs of Berlin porcelain, similar to those formerly employed by 

 CHAPPUIS and HARKKH, were kindly ordered for us by Dr. HOLBORN T at the Berlin 

 factory. They were made in manner described by HOLBORN and DAY in their paper 

 in ' Wiecl. Ann.,' vol. 68, pp. 8:10, 831. 



For attachment of the reservoir to the platinum capillary lending to the manometer 

 joints made by sealing-wax or cement are too treacherous to be trusted for long 

 periods, so the joints were in all cases made in the manner shown in fig. 2. The 

 brass washer soldered on to the platinum capillary is clamped down tight against the 

 ground end of the porcelain or glass neck of the reservoir by a hollow cap which 

 screws on to the two halves of a split brass collar, a thin leather washer, on which is 

 smeared a little marine glue, being interposed to make the joint. The collar is 

 prevented from sliding upon the neck by fitting closely into a rounded channel cut 

 into the glass or porcelain by a small emery wheel. This form of joint has given 

 perfect satisfaction. 



being lost through the plug being forced outward by the mercury pressure. Although the possibility of 

 the recurrence of this accident was avoided by the subsequent addition of a spring frame arrangement 

 embracing the plug of the tap, it was found extremely difficult to arrange the lubrication to be free 

 enough to permit of its constant use for shutting oft' communication between the thermometer and the 

 bulb and at the same time to prevent very slow inward leakage of air into A through the tap when the 

 bulb was under low pressure. The apparatus was therefore modified by putting into tube A a plain steel 

 tap and introducing a smaller and perfectly ground three-way tap into the side tube, where it was not 

 needed after the bulb was once filled. Under these conditions the whole .arrangement could be kept 

 perfectly gas tight. 



2 Y 2 



