352 
DR. J. T. BOTTOMLEY AND MR. F. A. KING ON THERMAL 
enclosure was beaten into shape out of a piece of thick sheet copper. It has a brass, 
cup-shaped flange, b, brazed on to the neck n, and the whole is thoroughly tinned both 
inside and outside, to make sure that every crack or pin-hole shall be filled in and the 
enclosure made vacuum tight. A seamless brass tube, cc, about 1^ centims. in internal 
diameter, has the corresponding flange, j, brazed on to its lower end, and has a collar, 
e, with a U -groove turned in it, soldered to its upper end. When the enclosure is 
brought up into position, the shoulder of the upper flange, j, fits into the cup of the 
flange on the enclosure, and the two are firmly clamped together by the four set¬ 
screws ^ The joint is then made vacuum-tight by pouring molten fusible metal into 
the cup, x, the flanges having been previously prepared by tinning them with fusible 
metal. * Fusible metal was used on account of its low melting-point, as the prolonged 
heating necessary to make a good soldered joint of such extent has a tendency to 
tarnish the highly polished silvered globe when it is suspended within the enclosure. 
A piece of wide glass tubing, tt, was ground to fit the groove in the collar e ; and, 
when all other preparations were complete, the joint was carefully sealed with 
“ Siegelwachs.” To prevent the “ Siegelwaclis ” from becoming brittle by the cold 
conducted up the brass tube cc, when the enclosure is immersed in a bath of liquid 
air, copper vanes were -clamped to the upper part of the brass tube to act as a 
radiator. A T-piece of quill tubing was sealed on to the glass tube t, to connect 
the enclosure to a pair of five-fall Sprengel pumps which are arranged to run con¬ 
tinuously. Interposed between the pumps and the enclosure is a M‘Leod gaugef for 
measuring the vacuum pressure; and attached to a three-way stopcock there is a 
bulb, l, filled with cocoanut charcoal, which can be put in communication with the 
enclosure. 
7. All the joints between the various pieces of glass apparatus are fused together 
with the blow-pipe, so as to avoid rubber, or other such connections. 
Th et ■mojunctio) is. 
8. For measuring the difference of temperatures between the globe and the enclosure, 
a pair of platinum to platinoid thermojunctions are used. These two metals give an 
excellent thermo-electric combination, as was shown in a paper on the subject by 
J. T. Bottomley and A. Tanakadate.| We have found them very satisfactory. 
The two thermojunctions are soldered into two little copper cylinders, oo, about 
* While working with fusible metal we have found that this material becomes springy at liquid-air 
temperature, and thus yields elastically to any changes in shape of the flanges. 
f An improvement was made in the present apparatus by placing the M‘Leod gauge as close as possible 
to the enclosure of which the vacuum pressure is to be measured by the gauge. We have found, from 
past experience, that it takes a considerable time for small differences of pressure to become- equalised in 
highly exhausted enclosures, especially when the rarefied gases have to diffuse through a length of glass 
tubing. 
i ‘Roy. Soc. Proc.,’ vol. 46, 1889, p. 286. 
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