62 Mr. A. Stansfield on some Improvements in 



air-thermometer. The present paper describes the Roberts- 

 Ansten recording pyrometer, and the changes which have 

 been made in view of the above objects, together with the 

 precautions which should be observed in order to avoid errors 

 in the use of the thermo-couple. It also contains an account 

 of the results that have been obtained, and the bearing of 

 these results on the theory of the thermo-couple. 



The Roberts-Austen Recording Pyrometer. 



The thermo-couple has been used in pyrometry in two 

 ways : — (1) by measuring theE.M.F. of the heated couple by 

 a null method, and (2) by observing the deflexion of a gal- 

 vanometer to which it is connected. 



In the recording pyrometer of Roberts-Austen * the latter 

 method is adopted ; the thermo-couple is connected to a 

 galvanometer the deflexion of which is recorded on a moving 

 photographic plate. The resistance of the galvanometer is 

 sufficiently large to render changes in the resistance of the 

 thermo-couple unimportant, and as the instrument is calibrated 

 directly by observations of known melting or boiling tempera- 

 tures, it is not necessary to assume any definite relation be- 

 tween the current and the deflexion of the galvanometer. 

 This form of pyrometer is extremely convenient and useful, 

 and the photographic records taken by its aid of the cooling 

 of an alloy of two or more metals frequently show small evo- 

 lutions of heat which could not easily be detected by direct 

 observation. These evolutions of heat, even when extremely 

 small, are of great importance in studying the constitution of 

 alloys. A few records obtained in this way are given in 

 fig. 2, which is a reproduction of a photographic plate 

 on which several records have been taken in succession. 

 Three of these, B, C, and D, represent the cooling of alloys of 

 copper and tin f containing respectively 50, 55, and 45 per 

 cent, of copper. 



In these allo}'s, most of the halts in the cooling-curves 

 represent the solidification of eutectic alloys. The curves A 

 and Z represent the cooling and solidification of aluminium 

 and of zinc, and the known melting-points of these metals 

 have enabled a temperature-scale to be constructed for use 

 with the other records. 



In this form of recording pyrometer the accuracy of the 

 record depends on the constancy of the galvanometer, as well 

 as on that of the thermo-couple itself, so that another element 



* Proe. Roy. Soc. 189], vol. xlix. p. o47. 



t Allovs Research Committee, 3rd and 4th Reports : Proc. Inst. Mech. 

 Eng. 1895, p. 269, and 1897, p. 67. 



