345 



microscope, provided with a micrometer of (lie niuvable-cross-liairtype 

 with divided cylinder: the diameters were measured in teji or twelve 

 directions, the squares of these numbers added, the total amount 

 divided by the number of measured diameters, and the square root 

 from this value considered as the true value for 2r. With regard 

 to the measurements to be made at extreme temperatures, and on 

 account of the fact, that a whetting of the capillary tubes appeared 

 to be necessary almost after every set of measurements, no tubes 

 with a radius of less than 0.040 cm. were used during these 

 investigations. 



The platinum-rhodium-tuhe ends at its upperside in a carefully 

 smoothed, polished brass disc; the tube of the adjustment-apparatus 

 R possesses just such a smoothed circular brass-plate. As a washer 

 between the two discs, a very thin ring of mica is used ; the capillary 

 tube is screwed against the end of the apparatus li, and both tubes 

 are then drawn together by the aid of two steel-keys, till the discs 

 are firmly pressed against each other: in this way an absolutely 

 gas-tight connection is obtained. This closure appears to be a very 

 perfect one, and if it is often controlled, no leakage needs to be 

 feared from this source. 



§ 9. The temperature-measurements were made with our calibrated 

 thermo-couples and compensator-equipment, in the w^ay always used 

 in this laboratory ^). Originally it was planned to connect the platinum- 

 wire of the thermocouple directly to the end of the platinum-rhodium- 

 tube by means of the oxygen-flame ; this tube then being considered 

 as the positive end of the thus obtained thermo-couple. However 

 the perfect isolation of the tube at very high temperatures appeared, 

 to be a serious obstacle ; so the idea was abandoned, and the usual 

 thermo-couples, provided with isolating MARQUARDT-capillaries, was 

 fixed to the platinum-rhodium-tube by means of fine platinum-wires; 

 at the other end they were connected with the ice-box Y (fig. la). 

 The wires of the element are bare over a distance of about 5 cm. 

 from the junction; this point lies in the same horizontal plane as the 

 lower end of the capillary tube, thus being in immediate vicinity 

 of its opening. Of course all platinum parts within the furnace 

 (crucibles, coils, etc.) need to be made from iridiumfree platinum, 

 to prevent contamination of the thermo-couples as much as possible. 



§ 10. The adjustment of the capillary tube with respect to the 



i; F. M. Jaeger. Eine Anieilung u. s. w. (1913), vid. p. 16—24. 



