240 Dr. H. A. Wilson on the Velocity of Solidification 



copper wires led to a very dead-beat mirror-galvanometer 

 read by a telescope and scale. A derlexion of about 3 millim. 

 divisions was obtained for a difference of temperature of 

 one degree between the couple and the mercury cups, so that 

 A of a degree could be measured easily. 



The salol was first melted by means of hot water, and then 

 the U-tube was fixed in the bath and the couple put halfway 

 down one limb. When the salol had taken up the tempe- 

 rature of the bath, solidification was started in the other limb, 

 and the galvanometer observed as the surface of separation 

 passed the couple. A millimetre-scale was fixed to the 

 U-tube, so that the temperature of the couple and its position 

 with respect to the surface of separation could be observed 

 simultaneously. The following series of observations were 

 obtained in one experiment: — 



Diameter of tubing, 0*5 centim. 

 Temperature of bath, 3l°"5 C. 

 Melting-point of the salol, 41° C. 



Distance of the Temperature-Difference 



Uoaple from the Sol id. indicated by the Couple, 



mms. 



5-6 



5-1 . 



4-7 0-2 



2-8 0*9 



2-3 1-2 



1-4 1-5 



1-0 1-9 



0-5 2-4 



0-4 3-0 



0-0 3-3 



-0-8 2-7 



-2-0 0-5 



The velocity of the boundary in this experiment was 

 1*3 millim. per minute. The maximum, temperature was 

 always indicated in the time during which the couple was 

 partly in the solid and partly in the liquid. The twisted 

 wires formed a couple about 0*4 millim. in diameter ; so that 

 in the experiment just described this time was about 18 seconds. 

 The following table gives the maximum rise of temperature 

 observed with the same tube at several temperatures. 



