Physics. — ''Further experiments with liquid helium. R. On the 

 electric resistance of pure metals etc. XI. Measurements con- 

 cerning the electric resistance of ordinary lead and o f uranium 

 lead below 14° K." (Comm. N». 1606 from the Physical Labo- 

 ratory at Leiden). By Prof. H. Kamerijngh Onnes and W. Tuyn. 



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



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



In Comm. N°. 133^/ § 13/? we reported that "Kalilbanm" lead 

 became superconducting at the boiling point of liquid helium, and 

 remained so at 4,°3 K., the highest temperature attainable with the 

 usual cryostat for liquid helium; in ^ 15 of the same Comm. from 

 the threshold value of the current at 4,"25 K. the vanishing point 

 temperature was estimated at about 6° K. The object of the invest- 

 igation described below was to establish the vanishing point tempe- 

 rature of lead more accurately, as well as to trace the ditference 

 in the vanishing point temperature of lead and uranium lead {Ra G) 

 and to follow the course of the change in the resistance of lead 

 with the temperature above the vanishing point, if possible up to 

 14°,0 K, the lowest liquid hydrogen temperature. Regarding a possible 

 difference of vanishing point temperature for isotopes it seemed not 

 impossible that the occurrence of the su[)erconductivity might be 

 influenced by the mass of the nucleus. ^). 



For the preparation of the resistances we used "Kahlbaum" lead 

 and uranium lead {Ra G), of which Prof. Hönigschmid of Vienna 

 very kindly put 16,5 gr. at our disposal; (he atomic weight of 

 ordinary lead from non-radio-active sources is 207,20, that of Ra G 

 from Bröggerit used is 206,06 *). Wires were drawn from both kinds 

 of lead and resistances prepared from them in the manner described 

 in ^ 1 of Comm. N°. 160^/; the chemical pioperties of the metal 



^) Concerning the properties of isotopes see the article by K. Fajans in the 

 Elsler-Geitel-Festschrift (Vieweg) and the Presidential Address to the American 

 Association at Baltimore, Dec. 1918 by T. W. Richards. 



') According to a letter from the firm of May 17th, 1916, "Kahlbaum" lead 

 contains a trace of Gu and Fe, the total impurity is less than (),017o; in a letter 

 of Dec. 8th, 1916 they give a more precise calculation of impurity : 0,002% Gu 

 and Fe. For an account of the atomic weight of lead isotopes cf. F. W. Aston 

 "Isotopes", London 1922. 



