SOSMAN AND HOSTETTERI A VACUUM FURNACE 279 



20 rhodium. 2 The tube is 15 mm. inside diameter and 200 mm. 

 long, with walls 1 mm. thick. The current is carried in by water 

 cooled terminals; the upper terminal is fixed to the furnace jacket, 

 while the lower terminal moves with the expansion and con- 

 traction of the furnace tube. The cooling water is led into the 

 lower terminal by lead pipes, and the current is carried by a 

 sheaf of flexible copper strips. The lower leads are insulated 

 from the furnace jacket by the stone base of the jacket, and are 

 prevented from accidentally touching the walls bj^ mica sheets 

 around the inside of the jacket. 



The platinum-rhodium alloy has the great advantage over 

 pure platinum of great mechanical strength and stiffness. Plati- 

 num-iridium alloys are even stronger mechanically, but the vola- 

 tility of the iridium forbids its use where thermoelements are 

 to be used. The stiffness of platinrhodium is shown by our 

 experience with the nitrogen thermometer, which had a 200 cc. 

 bulb made of the alloy containing 20 per cent rhodium. At 

 1100°, one atmosphere outside with about 200 mm. pressure 

 inside produced no observable change in the volume of the bulb 

 after cooling, although a difference of 0.05 cc. or 1 part in 4000 

 could have been easily detected. The strength of the platin- 

 rhodium tube was put to the test when during some measure- 

 ments at 1300° a glass connection was broken, allowing air at 

 atmospheric pressure to rush into the inside of the furnace tube 

 while the outside was still evacuated; we could detect no bulging 

 of the furnace tube resulting from the accident. 



The lower end of the platinrhodium tube is extended by a 

 steel tube, capped with a screw cap and sealed with kollolith.^ 

 The upper end is likewise extended by a steel tube into which 

 a glass tube is sealed, by the method of Kraus. This is an 

 extremely useful type of joint, now being used in the manufac- 

 ture of X-ray tubes, and should come into much wider use in 

 vacuum apparatus. 



^ The tube was made by Dr. Heraeus of Hanau, to whom we have several times 

 had occasion to express our indebtedness for his interest in the forwarding of 

 high temperature investigations. 



^ A Canada balsam substitute of constant melting point and low vapor pres- 

 sure, made by Voigt & Hochgesang. 



