504 Professor W. C. Roberts- Austen [Feb. 5, 



Pouillet (1836) one of platinum, and, Deville and Troost, in a truly 

 splendid series of investigations, adopted bulbs of porcelain, with 

 iodine vapour as the elastic fluid. They ultimately reverted to the 

 use of air. 



You will remember that old mercurial thermometers had much 

 information, supposed to be useful, engraven on their scales, and 

 such statements as " water freezes," " water boils," " blood heat," 

 " fever heat," " summer heat," were considered indispensable. It is 

 by exposure to known temperatures that a thermoscope can be con- 

 verted into a pyrometer for measuring intense heat ; and the air or 

 gas thermometer has, in the hands of Deville and Troost, rendered 

 excellent service by enabling such gradations to be effected. The gas 

 thermometer is not, in itself, a handy appliance, for it requires much 

 subsidiary apparatus, and elaborate corrections of various kinds have 

 to be introduced into the numerical data it affords ; but it has given 

 many fixed temperatures — such as melting-points and boiling-points 

 of elements, and of compounds — which may safely be made use of in 

 graduating pyrometers. For very high temperatures, 900° C. and 

 over, we rely on the excellent work of M. VioUe * on the specific 

 heats of platinum, silver, gold, palladium, and iridium, which have 

 enabled the melting-points of the respective metals to be calculated. 



The determinations of temperatures between 300° and 1000°, 

 which are now generally accepted, also rest upon data accumulated 

 by the aid of the air thermometer, which has thus enabled the 

 graduation to be effected of instruments widely differing from it, 

 that can be trusted to give rapid and accurate indications in daily 

 use. I can only bring before you two of the many kinds which have 

 been devised ; they are, however, by far the best that are available, 

 and for the determination of temperatures up to the melting-point 

 of platinum, leave little to be desired. 



(1) A pyrometer which depends on the increase in the resistance 

 of a heated conductor through which a divided electrical current is 

 passing ; and 



(2) One in which the strength of an electric current, generated 

 by the heating of a thermo-junction, is used as a measure of the heat 

 applied to the thermo-junction. 



The principle of the electrical resistance pyrometer was indicated 

 by Sir "William Siemens (' Collected Papers,' vol. ii. * Electricity,' 

 p. 84, 1889) in a letter addressed to Dr. Tyndall, dated December 

 1860, and the nature of the instrument may be made clear by the 

 accompanying diagram, Fig. 1. A divided current passes from the 

 battery B, to a platinum wdre, C, coiled round a clay cylinder, and 

 to a resistance coil, R. At the ordinary temperature the resistance 

 of the platinum coil is balanced by the standard resistance R. If, 

 however, the platinum coil be heated, its resistance will be increased, 

 and, this increase of resistance, which can be measured in various 



* ' Comptes Rendus,' vol. Ixxxix. p. 702, 1879; vol. xcii. p. 8G6, 1881. 



