677 



increase of resistance iiiul decrease of tlie temperature coefticieiit, is 

 removed in gold and platinum for instance, by heating. With tin, 

 on the other hand, heating is injurious, it causes the resistance to 

 increase '), moreover, it causes thin wires to go into angular forms -'). 

 The threads we used were, therefore, not heated after being worked, 

 and showed regular curvatures when bent. 



ff Potential phejiomena in the super-conducting state. The following 

 observations allow us to judge of the highest limit of the possible 

 micro-residual resistance, and of the potential differences above the 

 threshold value of the current density just below the vanishing point. 

 They were made with a branching tin wire exactly like the one 

 used in the experiments with mercury of Table IV and V in Comni. 

 VI of this series, § 6 and 7. The resistance consisted of a principal 

 wire Wf 4 m. long, and mainly 0.0097 mnr. section ') with two 



^) According to Tammann and his school, the crystals are shattered by wire 

 drawing, and arranged in such a way that in the cases meant the resistance 

 increases. By heating, larger crystals are again formed, and the resistance resumes 

 its original value, hi the investigations of Kamsrlingh Onnes and Clay, (Gomm. 

 N". 996 § 4, June 1907), it is pointed out that the additive resistance of platinum 

 and gold wires is always found greater by continued drawing even after heating 

 to glowing. We attributed this to the acquiring of admixtures through the drawing. 

 In gold it is possible to test for such small quantities of admixture as are here of 

 importance. In gold wires carefully drawn by Heraeus (Comm. N'. 99c § 2, June 

 1907), under repeated treatment with acids, larger quantities of admixtures were 

 found in proportion as the resistance fell less at reduction to hydrogen tempera- 

 tures. At the same time it is possible that the drawing itself has an influence. 

 Henning (Ann. d. Phys. 1913j, thinking as we do, attributes the difference found 

 with his platinum thermometers in the temperature coefficient from that found by 

 us, to a larger amount of admixtures in our thermometer. The difference becomes 

 greater still, when we consider that Henning's wire (0.05 mm.) was drawn out 

 further than ours (0.1 mm.) (which is of importance in the application to thermo- 

 metry). As mentioned above and as we found confirmed in comparing the wires 

 Ptj (0,1 mm.) and Pt^ (0,05 mm.), thinner wires fall less in resistance, a result 

 by which we also explained, 1. c, why Holeorn's thick wires (0.2 mm.) showed 

 a greater fall than ours. Our wires were at the time most carefully drawn by 

 Heraeus from the purest platinum supplied by him. The platinum obtained by 

 Heraeus later on may have been even purer. Improvement may also have been 

 made in the method of drawing the wires. 



^) Where broken, tin wires exhibit comparatively large crystals. See also 

 § 15 note 1. 



*) In this investigation the section is deduced from the length of the wire and 

 the resistance at ordinary temperature. We only ascertained, whether this agreed 

 approximately with the result of direct measurement. The values given are there- 

 ore only to be considered as rough mean values. 



