675 



sudden disapparaiice was observed wlieii (he leiiii)era(in-e fell (o 



3°,806 K. (boiling under 47 cm. mercury pressure). At 3°. 82 K. 



the resistance of one was still 0.0J83 S2, of the other 0.00584, at 



3°. 785 K. of both < 10-^ ^y, in this case too the highest limit for 



a possible micro-residual resistance was thus very low. We may put 



w 

 -i!:iA<io-7. 



^27S0K. 



Besides the sudden disappearance of the resistance of the wire, 

 we also observed, as in the mercury thread, that for each tempe- 

 rature below the vanishing point a threshold value for the current 

 density ^) determined by this temperature, fin the case of the last 

 mentioned wire the threshold current was 0.28 amp. at 3°. 785 K.) 

 could be fixed, below which the current passes without any perceptible 

 fall of potential, and above which it is accompanied by potential 

 phenomena, which (see § 14) increase rapidly with the increase of 

 the excess of the current above the threshold value. In a word, the 

 tin wire behaves below the vanishing temperature of the tin, 3°. 8 K., 

 qualitatively precisely the same as a thread of mercury below the 

 vanishing point of that metal. 



/I Lead of Kahlbaum, made into a wire in the same way as the 

 tin, 1.5 m. long and 10.8 S2 resistance at ordinary temperature, 

 when it w^as immersed in liquid helium appeared to be super- 

 conducting, without the necessity of reducing the pressure at which 

 the helium boiled. When the temperature was raised as far as the 

 cryostat permitted, that is to 4°. 29 K. (the pressure was raised 11 cm. 

 mercury above 76 cm.) the lead remained super-conducting. The 

 temperature at which the ordinary resistance of the lead disappears 

 will probably, as indicated in § 15, not be far above the boiling 

 point of helium. 



Whether this disappearance, as with mercury and tin, also takes 

 place suddenly, has yet to be investigated. For temperatures below 

 14° K., where lead has still a relatively high ordinary resistance, 

 and above 4°. 3 K. where it has disappeared, we do not yet possess 

 a satisfactory cryostat. At the temperature just mentioned of 4°. 29 K. 

 we found that the threshold value of the current was not yet reached 

 at 1.3 amp. 



1) Concerning the dependence of the Ihi-esliold value upon the dimensions of 

 the wire and the conditions under which the heal is given off, further investi- 

 gation is needed. 



