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VI. On the Practical Measurement of Temperature : Experiments made at the Cavendish 
Laboratory, Cambridge. 
By H. L. C Allen])ar, B.A., Scholar of Trinity College, Cambridge. 
Communicated by J. J. Thomson, F.R.S., Cavendish Professor of Experimental 
Physics in the University of Cambridge. 
Received June 9,—Read June 10, 1886.* 
[Plates 11-13.] 
General Account of the Experiments, and Discussion of the Results. 
In view of the enormous discrepancies at present existing in estimates of high 
temperatures, it is exceedingly desirable that strictly comparable thermometric 
standards should be issued by some recognised authority. 
Professor J. J. Thomson, in the course of a conversation which I had with him 
towards the close of 1885, suggested that such standards could be issued in the form 
of platinum wire, the change of electrical resistance with temperature being deter¬ 
mined by comparison for each specimen before issuing. The object of the present 
investigation was to test whether, in spite of the B. A. report on the Siemens pyro¬ 
meter (1874), pure platinum wire might not be possessed of the necessary qualifica¬ 
tions for such a standard. 
Let us first consider what qualifications are necessary. 
(I.) Such a standard should always give the same indication at the same tempera¬ 
ture, should be free from secular change of zero, and should possess the widest possible 
range. 
(II.) It should be portable and readily copied, so that standards might be multi¬ 
plied, and, if the original were lost, it might be replaced by means of its copies. The 
copies should be capable of accurate comparison and verification. 
(III.) It would be convenient if it agreed very approximately with any standards 
already in use, and if, failing this, the relation between the new and pre-existing 
standards could be accurately ascertained. 
(IV.) It would be a great additional advantage if, besides being useful as a 
standard, it could also be applied directly to all kinds of practical investigations. 
With regard to the first point (I.); the self-consistency of the platinum thermometer 
* The portions enclosed in square brackets have been added or altered subsequently, June, 1887. 
MDCCCLXXXVII.—A. Y 
18.8.87 
