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Physics. — “Further experiments with liquid helium. C. On the 
change of electric resistance of pure metals at very lom tem- 
peratures ete. IV. The resistance of pure mercury at helium 
temperatures.” By Prof. H. KAMERLINGH ONNms. Communication 
N°. 120° from the Physical Laboratory at Leiden. 
§ 1. Lntroduction. Since the appearance of the last Communi- 
cation dealing with liquid helium temperatures (December 1910 
liquid helium has been successfully transferred from the apparatus 
in which it was liquefied to another vessel connected with it, in 
whieh the measuring apparatus for the experiments could be immer- 
sed — in fact, to a helium cryostat. The arrangements adopted for 
this purpose which have been found to be quite reliable will be 
described in full detail in a subsequent Communication. In the 
meantime there is every reason for the publication of a preliminary 
note dealing only with the results of the first measurements made 
with this apparatus, in which I have once more obtained invaluable 
assistance from Dr. Dorsman and Mr.G. Horst. These results confirm 
and extend the conclusions drawn from the previous experiments 
upon the change with temperature of the resistance of metals. 
Moreover, it was, in the first place shown that liquid helium is an 
excellent insulator, a fact which had not hitherto been specifically 
established. This was of importance since the resistance measurements 
were made with naked wires, a method that is permissible only 
if the electrical conductivity of the liquid helium is inappreciable. 
§ 2. The resistance of gold at helium temperatures. In the second 
place a link in the chain of reasoning which I adopted in $ 3 of 
Communication N°. 119% to show that the resistance of pure gold 
is already inappreciable at the boiling point of liquid helium has 
been put to the test by determining the resistance in liquid helium 
of the gold wire Awyz7, which was then estimated by extrapolation 
on the analogy of the platinum measurements. Within the limits of 
experimental error, which are indeed greater for the present experiment 
than was the case for the others, that value is now supported by 
direct measurement. The conclusion that the resistance of pure gold 
within the limits of accuracy experimentally obtainable vanishes at 
helium temperatures is hereby greatly strengthened. 
§ 83. The resistance of pure mercury. The third most important 
determination was one of the resistance of mercury. In Communi- 
