104 Profs. Dewar and Fleming on the 



of which the electromotive force was being measured was con- 

 nected in series with the galvanometer, and joined in between 

 the slider P and the end of the resistance R 4 . In this case 

 the fall of potential down the slide-wire was measured as 

 usual by a Clark cell Ck. In order to make the changes of 

 connexion rapidly from bridge to potentiometer, a seven- 

 cup mercury switch S was employed, so that by shifting a 

 pair of copper forks all the necessary changes of connexion 

 were made instantly. 



6. The operations of measurement were then as follows : — 

 One set of the thermo-junctions being immersed in melting- 

 ice, the working thermometer, with the other set of thermo- 

 junctions packed inside it, was placed in a small copper tube 

 closed at the bottom end, not touching the tube, and this 

 was filled full of melted paraffin- wax. The copper tube was 

 then placed in a test-tube and the whole filled with melted 

 paraffin- wax, to between the copper tube and the glass test- 

 tube. When the wax was solid, the mass containing the 

 thermometer-wire and junctions enclosed within it was 

 cooled down by the employment of considerable quantities of 

 liquid air to a temperature of about —200°. As soon as the 

 temperature had become constant, the mass of paraffin was 

 lifted out and placed in a vacuum-jacketed vessel that it might 

 become heated up by conduction and radiation very slowly. 

 A series of observations was then taken for each junction in 

 rotation, measuring first the temperature of the junctions by 

 the thermometer-wire resistance, then changing the con- 

 nexions, and measuring the electromotive force of the couple, 

 then changing back again to the bridge arrangement to get the 

 temperature of the junction again; and so on for each couple 

 in rotation, as the mass of paraffin slowly warmed up. In this 

 manner a series of observations was obtained w T hich, when 

 reduced, gave the electromotive force of the couple in absolute 

 C.G.S. units and the standard platinum temperature (pt) of 

 the variable junction at that instant, the other junction being 

 kept at 0° C. all the time. 



By the employment of carbonic acid and ether and a 

 paraffin-oil bath the series of observations for closely adjacent 

 temperatures was continued for each thermo-couple from the 

 boiling-point of liquid air up to the boiling-point of water. 

 In addition to this, very careful observations were taken of 

 the E.M.F. of all the couples at fixed stationary temperatures, 

 such as that of the boiling-point of liquid oxygen, the melting- 

 point of solid carbonic acid, the melting-point of ice, and the 

 boiling-point of pure water, and the intermediate observations 

 were made in some cases with, the temperature of the junctions 



