Practical Thermometric Standard. 529 



It may reasonably be expected tbat, in the course of tbe 

 next few years, by the application of electric heating and 

 other improvements, the results attainable with the gas- 

 thermometer will be greatly improved, but in the meantime 

 it must be admitted that it cannot approach the platinum- 

 thermometer either in point of accuracy of measurement or 

 facility of application. The latter instrument, for instance, 

 working in conjunction with a purely automatic indicator, 

 which records the temperature in pen and ink on a scale of 

 4 millim. to 1° C, is capable of repeating the determination 

 of the F.P. of silver an almost unlimited number of times 

 with a maximum error of one or two tenths of a degree. 



On the Choice of a Practical Standard. 



In the paper (communicated to the Royal Society in June 

 1886) in which the proposal for the adoption of the platinum 

 resistance-thermometer as a practical standard was originally 

 made, the following conditions were laid down as desirable 

 qualifications for such a standard : — 



(I.) Such a standard should always give the same indication 

 at the same temperature, should be free from change of zero, 

 and should possess the widest possible range. 



(II.) It should be portable and readily copied, so that 

 standards might be multiplied, and, if the original were lost, 

 it might be replaced by means of its copies. The copies 

 should be capable of the most 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 directly applied to 

 all kinds of practical investigations. 



In the light of the experience which has since been 

 accumulated on the measurement of high temperatures, it may 

 be instructive to reconsider the question of a practical standard 

 from the point of view of these qualifications : — 



(I.) By the first qualification of great constancy over a wide 

 range, we are practically limited to the four methods based 

 on the physical properties of the platinum group of metals, 

 which may be called the " Platinum Methods/' Some typical 

 results obtained by these methods are collected in Table II. 

 The methods are : — (1) The Expansion Method, based on the 

 expansion of platinum, (2) The Calorimetric Method, based 



