1424 
If we take the last described experiments together, we have 
been able by them on the one hand to raise the current density 
to the enormous value of about 1000 amp. per mm?, without 
any heat being developed in the wire. This threshold value for 
Nn 
TABLE VI. | 
_ Potential difference at the extremity of a mercury 
thread carrying a current below 4.°2 K. 
zr? = 0.0025 mm? 
| 
current density | potential diff. 
Temp. in amp. per mm.” | in microvolts 
2°.45 K. | 944 | < 0.03 
> | 1024 | 0.56 
| > | 1064 | 15 
| > | 1096 | 6.3 | 
| > 1120 | very large 
| 
the current density brings the highest limit for the possible resistance 
of mercury in the peculiar condition into which it passes below 
4°.19 K. and particularly when it is cooled to 2°.45 K. still further 
back, and the ratio of the resistance at 2°.25 K. to that of solid 
< 2A0-%, 
Wao45 K 
mercury at 273° K. becomes 
2730 K 
On the other hand it is proved that the development of heat 
which appears at a still higher strength of current, has its origin 
in the thread itself. 
§ 8. Injluence of the current density upon the manner in which 
the resistance in mercury threads disappears. What has been related 
above can all very well be reconciled with the view (see § 5) that- 
the disappearance of the ordinary mercury resistance at 4°.19 K. 
occurs quite suddenly, and in a thread that has been cooled to below 
that temperature, as soon as the “threshold value” of the current 
density is exceeded, somewhere heating occurs which carries the 
thread at that place to above that temperature, at first over a scarcely 
perceptible length but at higher currents over a rapidly increasing 
distance, by which ordinary resistance is generated in this part of 
the wire. With these larger currents the thread then comes in astate 
