83 



The frame BG is provided with a pall bound with silk, which 

 prevents the viscosimeter going- bc3ond the vertical position. The 

 turning' takes i)lace in a phxne parallel to the long axis of the ellipse. 



For the observation of the viscosimeter there ai-e two view tubes 

 KB fastened into the cover in the direction of the short axis of the 

 ellipse, which reach below the upper bulb, (fig. 1 and 3). (In tig. 2, 

 part of one of the tubes is shown). These tubes are closed air-tight 

 above the cover bj thick glass covering plates DP. The light of a 

 glowlamp is thrown upon the viscosimetei' b}^ the mirrors S^ and 

 ^Sj, it can be viewed by the mii'rors S^ and aS (fig. 1). The mirrors 

 S^ and aSj (tig. 7) ai-e attached to a copper cross which is soldered 

 on to a hoop of copper wire A", resting upon the bottom of the view- 

 tube, and by which the mirrors can be easily adjusted. When they 

 are in the right position the wire is fastened with paraftine. Opposite 

 to the mirror the view-tubes have a side opening ground flat, on to 

 which a covering glass is fastened with tish-glue. This connection 

 remains firm even in liquid air. By this means the light falls through 

 a plan-parallel layer of liquid, so that the bulb does not ai)pear 

 distorted. To avoid the glue being exposed to pressure, there are 

 openings higher up in the view-tubes (fig. 1). By moving the viscosi- 

 meter a short distance up and down with the handle TS the mark 

 above the bulb or the beginning of the capillary tube can be brought 

 into view. The spiral spring remains tight the whole time. 



In addition the vessel contains : 



1". an electro- magnetic stirring pump RP (fig. 2 and fig. 3 ; in 

 fig. 1 a part of it). The iron core V is moved up and down by an 

 electro-magnet round the tube RP^. The bottom of the cylinder /^/-'j 

 and the piston are provided with suitable valves, turning on an axis 

 represented in fig. 3 by a double dotted line. The shape of the 

 cylinder is here shown also. 



2". a floater which indicates the height of the boiling liquid. 



3". a platinum resistance-thermometer, of which only the tube 

 sealed into the cover is shown : TH (fig. 2 and 3). 



4". a supply tube for the liquid TV, which is closed after tilling. 



5°. a tube A V to lead off the vapour, when working at low 

 pressure. 



6". three tubes B^, B.^ and B^ (fig. 3) which fill up the super- 

 fluous space, to save liquid and trouble. 



When tested this cryostat appeared to be in every way satisfac- 

 tory, the distinctness of the readings left nothing to be desii-ed. At 

 the same time the volume of the butane was so much diminished 

 b^' contraction that it was difücult to get sufficient pressure, when 



6* 



