i 5 8 TRANSPIRATION AND ASCENT OF SAP ch. 



The specific resistance of eureka at is given as 



47*4 microhms; its variation at 20 per 1 as - 0048 per cent. 



The construction of the eureka-copper thermocouple is 



simplicity itself. To each end of the silk-covered piece of 



eureka wire, about 1 m. long, a con- 

 venient length of the copper lead is 

 soldered. The eureka wire I made 

 use of had a diameter of 01 9 mm., 

 and a resistance of about 16 5 ohms 

 per metre. The soldered junctions 

 between the eureka and copper may 

 be'neatly made by stripping a few 

 millimetres of the ends of each from 

 their silk coverings and dipping the 

 bared tips into a solution of resin in 

 spirit. After this treatment, if the 

 ends in contact with one another 

 are immersed in a tiny drop of 

 molten solder, a very compact and 

 good junction is made. 

 Description of the apparatus. 

 To accommodate the couple to the 

 apparatus, the eureka wire before 

 soldering was wound on a cork 

 support (Fig. 25 s), leaving some 

 20 cm. of each end free. This cork 

 support forms a connecting-piece 

 between two drawn pine rods (p and 

 r, Fig. 25) which are destined to carry the junctions 

 and to keep them in position, one in a test-tube {a) con- 

 taining the fluid to be examined, and the other in a similar 

 tube (6) containing distilled water. 1 



1 If a finer wire is used, the resistance may be disposed of by winding 

 it round the lower end of the rod p, so that it remains immersed in the 

 freezing distilled water. This eliminates any change in the resistance due 

 to temperature fluctuation. 



Fi; 



c, 



