70 Mr. A. Stansfield on some Improvements in 



(2) To obtain the value of the potentiometer E.M.F. in 

 terms of the standard Clark cell by means of the 

 auxiliaiy potentiometer. 



(3) To calibrate the auxiliary potentiometer. 



(4) To read the temperature of the Clark cells by means 

 of the thermometer t . 



(5) To determine the relation between the deflexion of the 



galvanometer and the potentiometer-unit. 



Fig. 4 is a reproduction of a record, obtained in this 

 way, of the solidification or freezing of tin. The metal 

 was contained in a small clay crucible inside a cast-iron 

 chamber heated externally, so that very slow and regular 

 cooling could be obtained. 



The metal was stirred during the solidification in order to 

 promote uniformity of temperature and to prevent surfusion. 

 It will be noticed that the temperature remained practically 

 constant until the greater part of the metal had solidified. 



Records have also been taken on the same scale of the 

 freezing of metals having much higher melting-points, such 

 as gold or copper ; but the difficulty of obtaining a satisfactory 

 curve is greatly increased. 



Discussion of the Observations. 



In order to discuss the theoretical bearing of the readings 

 of the thermo-couples it is necessary to know the temperatures 

 to which they correspond. Although measurements of high 

 temperatures have been made with the air-thermometer, they 

 are not given in the present research. The results of different 

 experimenters have nevertheless determined the melting-tem- 

 peratures of silver, gold, and copper, with an error of probably 

 less than 10°; the relative temperatures being known much 

 more accurately. For the purposes of the mathematical 

 treatment of the results, the melting-temperature determina- 

 tions of Heycock and Neville may be taken. Although their 

 measurements of the melting- temperatures of silver, gold, and 

 copper depend on a somewhat severe extrapolation of a 

 formula, their determinations of the relative melting-points 

 of these metals are probably very accurate, and their results 

 agree fairly well with those obtained by the direct air-ther- 

 mometer observations of Holborn and Wien. 



In fig. 5 the results obtained by the author for different 

 thermo-couples are plotted in the usual way ; the co- 

 ordinates representing the temperature of the hot junction 

 (r), and the observed E.M.F. of the couple (E), the cold 

 junction being supposed to be at 0° C. 



Curves representing some of the observations of Barus, 



