996 



The adjustment-tube was manufactured from a tube which was 

 as free as possible from optical erroi'S : it did not give a difference 

 in the reading, when a kathetometer was pointed on the adjustment- 

 point / and the tube was interposed or removed. The steel piece 

 is taken long enough to be sure that the tube on the level where 

 the adjustment is made is not injured by the heating, when the 

 capillary is sealed on. 



To ensure uniformity of temperature of the mercury in the U-inhe 

 of the manometer — another condition foi* extreme accuracy — 

 the glass-part of the thermometer which contains mercury is now 

 so constructed') that it may be conveniently surrounded by a closed *) 



which is attached to the thermometer for this purpose is sealed in a mamier 

 which may be seen in figg. 1, and 3 and 4 after the piece has been brought into its 

 proper position. In the thermometer for helium-temperatures the operation could 

 be performed by means of a ground joint (see fig. 3 ('omm. No. 1246). 



The piece is attached to the glass in its proper position by means of a very 

 small quantity of sealing wax which has been previously freed of all vapours by 

 exhaustion with liquid air. The bottom face of the piece is ground flat at right 

 angles to the tube. It carries a point 'lb of a mm. long. 



') For the thermometer of helium-temperatures where very small dilferences of 

 pressure had to be measured a pasted down cardboard chamber, fitted on the 

 inside with copper screens, had been improvised for the same purpose. With the 

 present arrangement the difference in the readings of two thermometers placed at 

 different points in the chamber was less than 05 of a degree. The change during 

 a day was below -1 of a degree. 



2) A characteristic of this type of thermometer is that the whole can be easily 

 moved to a different position. This requirement has lost the special importance 

 which it had at the time, when the first type was developed and it was still so 

 much more ditticult to obtain absolutely constant low temperatures. 



At the same time it has still its great advantange and it was possible to retain 

 it in the new type. A movement up and down of the adjustment-space has become 

 unnecessary by the special arrangement for pressures below one atmosphere. 



The stand shown in previous firawiiigs has been replaced by a more satisfactory 

 one. Its base is now connected permanently with the levelling table on which it 

 is mounted. The rod has been detached from the base and rests on it in the 

 manner of the cone of the axle of a theodolithe resting in its conical seat (see 

 fig. 2). This arrangement is especially convenient, when several thermometers are 

 in use which it is desired to put by ready for use. Each thermometer then has its 

 own rod, to which it remains , attached while being kept ; when it is to be used 

 the rod is placed on to the common foot-stand. 



In order that the kathetometer needs only be turned to focus both tubes of the 

 manometer with equal sharpness, the manometer — like the barometer and standard- 

 metre — is mounted on a slide which can be moved in the direction of the 

 kathetometer-axis and the rod to which both arms of the manometer are attached 

 can turn about the cone which is attached to the slide. (For the finer adjustment 

 in this respect consult details in fig. 2). 



