606 Professor W. C. Moherts- Austen [Feb. 5, 



The necessity for working with small volumes of fused metals, 

 into which the tube of Callendar's pyrometer could not be plunged, 

 has led me to prefer to adopt a method that would be classified under 

 the second heading I have given. A very small thermo-junction 

 may, in fact, be employed in such cases. The use of thermo-junctions 

 for measuring high temperatures appears to have been suggested in 

 1826 by Becquerel, and adopted by Pouillet in 1836,* who advocates 

 the use of iron in conjunction with platinum ; but of all the varied 

 combinations of metals and alloys which have been tried from time 

 to time, that proposed by II. Le Chatelier possesses many advantages, 

 on which I have elsewhere dwelt.f It consists of a platinum wire 

 twisted at its end with a wire of platinum alloyed with 10 per cent, 

 of rhodium. Such a couple may be used for some time without 

 change of zero, and if the junction becomes injured it may be cut off, 

 and the severed ends of the wires may be twisted together again. I 

 am satisfied that it can afford comparative results which are accurate 

 to 1° at temperatures of over 1000°. The diagrams given later 

 (Figs. 4, 5, and 6) show the disposition of the apparatus. The spot 

 of light indicating the deflections of the galvanometer needle is 

 caused, for the illustrations of this lecture, to fall on to a graduated 

 scale 45 feet long on the wall of the theatre. The thermo-junction 

 has been calibrated with the aid of certain known temperatures, and 

 the long scale is inscribed after the manner of the old thermometer 

 scales, with certain fixed points, which are, of course, far higher than 

 those it was possible to indicate by the expansion of mercury in a 

 glass tube. (These fixed points were : — " water boils " (100°), " lead 

 melts" (326°), "zinc boils" (940°), " gold melts" (1045°), "palla- 

 dium melts" (1500^), "platinum melts" (1775°). On heating the 

 thermo-junction to bright redness in a Bunsen flame, the spot of 

 light moved rapidly to the point marked " zinc boils.") For labora- 

 tory experiments the scale is a short transparent one, rigidly fixed 

 in relation to the galvanometer. 



In leading up to the experiments which follow, in the course of 

 which metals will be exposed to high temperatures, I would remind 

 you thai if an ordinary thermometer be plunged into water which is 

 gradually losing its heat to a cold environment, the mercury will fall 

 until the water begins to freeze, but directly this happens the 

 mercury remains stationary until all the water is frozen ; so that if 

 the rate of fall be measured with a chronograph, there will be a 

 steady fall to the freezing point of water, then a long arrest, 

 followed by a renewed fall. If these readings be plotted, a well- 

 known time-temperature curve will be obtained. Exactly the same 

 effect is produced when a fluid metal " freezes," and before proceed- 

 ing further it may be well to determine experimentally the freezing- 



* ' Comptes Rendus,' vol. iii, p. 782, 1836. 



t British Associatiot: Lecture, 'Nature,' vol. xli. 1889, pp. 11-32; Report 

 Inst. Mech. Eng. Oct. 1891, p. 543. 



