( (^31 ) 



the adjustment of the bath to the required temperature (say at — 120') 

 was obtained within one hour after the [)unips in the cryogenic 

 laboratory had been set working. 



Instead of a resisrance thermometer, to regulate the temperature, 

 Ave used the thermoelement O, the protected junction being placed 

 at tiie side of the piezometer (comp. comm. 27 June '96) ; it is visible 

 through the window V\ (in plate IV). The electro-motive power of 

 the thermoelement is compared by means of the zero method with 

 that of a thermoelectric control element or a WKSTON-element. 



For the same difference of temj)erature the deflections on the scale 

 of the sensiti\'e galvanometer were almost as large as in the measure- 

 ments made with the resistance thermometer (comp, § f)\. An example 

 of the determination of the temperature is not necessary in addition 

 to Plate III. 



IV. ^4. permanent hatii of liquid nitrogen at ordinary or reduced 

 pressure. In Comm. 14 (Dec. '94) a short description was given of the 

 temperature steps obtained by means of circulations of methylchloride, 

 ethylene and oxygen. In connection with that description I mentioned 

 my intention of adding more circulations to those already existing 

 and said that I hoped to replace more and more parts of the existing 

 circulations by greater and to insert such technical apparatus as 

 should be found advisal>le so that the existing apparatus could be 

 used in the new circulations with [)ure or costly gases. An example 

 of this is the circulation of nitrogen added to the existing temperature 

 cascade, of which a description is now required by the completion 

 of some of the measurements rendered possible by it. For measure- 

 ments at temperature between — 195° C. and — 210° C. a nitrogen 

 is much to be preferred to an oxygen-circulation as the tension at 

 which the oxygen boils at — 195° is so small that accurate regulation 

 at constant temperature becomes very difficult. As the preparation of 

 pure nitrogen in such large quantities as a circulation requires 

 presents many difficulties, the compressor and the vacuum pump 

 must be suitable and efficient. These conditions are fulfilled by the 

 mercury and the auxiliary compressors which are generally used for 

 the compression of pure gas and which in the originally tempera- 

 ture cascade served for the oxygen circulation. However when the 

 BROTHERHOODcompressor (comp. Comm. 14 Dec. '94 and 51 Sept. '99) 

 could be used for the oxygen circulation in the cascade the3^ could 

 be used for the nitrogen circulation. 



The nitrogen is prepared from sodium nitrite. Besides being passed 

 through ferrous sulphate and sulphuric acid it is led over hot cop- 



