Physics. — "Further experiments witJi liquid helium. Q. On the 

 electric resistance of pure metals etc. X. M ea.'iurements con- 

 cerning the electric resistance of thallium in the temperature 

 field of liquid helium." (Comm. W. 160a from the Physical 

 Laboratory at Leiden). By Prof. H. Kamerlingh Onnes and 



W. TUYN. 



(Communicated at the meeting of October 28, 1922). 



^ 1. Object of the research. Method of preparing the resistances. 

 The place of thallium in the periodic system of elements, between 

 the super-conducting metals mercury and lead, made it seem pro- 

 bable that it would become super-conducting at helium temperatures. 



We had at our disposal only rods of thallium from Kahlbaum^). 

 From this Mr. P. J. v. d. Baan, instrumentmaker of the Phys. 

 Lab., extruded wires of 0.2 and 0.5 m.m. thickness; they were 

 bright at tirst, but quickly became tarnished and grey in colour. 

 At the distance of a few cm. from the ends of each wire a 

 second shoit wire was melted on in a small gas-flame; during 

 this process the thallium was protected from oxidation by a layer 

 of melted candle grease. The wire was then wound bifilarly upon 

 a porcelain tube with a double screw thread baked into it, (these 

 lubes were made by the Königliche Porzellan-Manufaktur, Berlin and 

 have been mentioned before in Comm. NV 152c § 2) and then the 

 four thallium ends were each soldered to a copper wire, previously 

 attached to the tube. The resistance thus prepared was enclosed in 

 a glass tube made by the chief glass blower of the Phys. Lab. Mr. 0. 

 Kessklring, in the following manner. The ends of this tube through 

 which the copper wires protuded were platinised, coppered, provided 

 with copper caps and sealed up (see also Comm. W. 133t/, p. 60). 

 To remove the oxidation layer on the Tl-wive the resistance was 

 rinsed through the opening at the other end of the glass tube and 

 dried by a moisture absorber and carbon tube; a tap attached 



') According to a letter from the firm the thallium contained the usual amount 

 of lead; about other impurities nothing was said. The same letter said that the 

 firm did not prepare any "extra" pure material. M. Levin (Z.S. f. An. Chem. 45 

 (1905), p. 31) states that KAHLBAUM-thaliium contains 99,91o/üT1, N. Kuenakow, 

 S. Zemczuzny and V. Tararin (Z S. f. An. Chem. 83 (1913), p. 200), only say 

 that they used pure Tl from Kahlbaum. 



