B44 



the tliennostiit and just behind the capillary tube C a glass-scale X 

 is introduced, which possesses a very accurate division in 0,2 mm., 

 by means of very fine lines (3 microns) made by the Compagnie 

 Génévoise. The scale is read with a telescope and ocular-micrometer 

 by the same tirra, and at a distance of about 2 Meters ; the enlarge- 

 ment is about 25. During the readings the motor /v must be stopped 

 for a moment, because even the slight vibrations are rather annoying. 

 The thermostat is wrapped with felt except for the narrow opening 

 needed for the readings. Behind the manometer a ground glass-plate 

 is applied, which is illuminated by two long, tube-straight-filament 

 incandescent-lamps, each of 50 candle power. Between the lamps and 

 the thermostat, a watertank with giasswindows, is introduced, to 

 prevent beat-radiation into the thermostat. With this mode of lighting 

 the fine divisions and the octane-surface are seen very distinctly', 

 without any observable parallax, against an illuminated background 

 and readings can be made with extreme accuracy, with the aid of 

 the movable cross-hair of the micrometer. However readings of less 

 than 0,1 mm. appeared to be unnecessary, because of the fact, that 

 the mean oscillations in the successive determinations of H, were 

 about ±0,05 mm. octane, the total uncertainty Iherefoi-e being about 

 0,1 mm. octane-pressure, or about 0,005 m.m. mercury-pressure. 



§ 8. To bring the capillary tube into a vertical position, a mer- 

 cury mirror was used : by means of the screws A^ and A^ (fig. ^b) 

 the position of the capillary tube is altered until its mirrorimage 

 will seem to be in a straight line ^vith the tube ; the position of P^ 

 and P^ is then fixed by means of the screw B. The capillary 

 tube itself was origiualiy made from purest, iridiumfree platinum ; 

 this however appeared to be too soft, and so an alloy with 10°/o or 

 207fl of rhodium was used afterwards. It is impossible to get any 

 suitable capillary tubes for ihis purpose from the shops in trade. 

 Therefore the rough cai)illary lube with its widened (ca. 6 mm.) 

 upper part, was purveyed by Heraeus ; the lower end was then 

 carefully turned off on the lathe to a conical and sharp edge, which 

 was once more whetted on an oil-stone, if necessary. 



With some practice in this way the repairs of the damaged or 

 worn capillary tubes can be accomplished within a relatively short 

 time; and it proved to be possible to obtain a cross section of the 

 tube, which in several directions did not deviate more than about 

 0,002 m.m. from a pure circular shape, while t!ie rim of the lower 

 end measured no more than 0,01 m.m. 



The cross section was determined bv means of a horizontal 



