284 Mr. N. Eumorfopoulos on the Determination 



The general results obtained so far are : — 



Rods. 



»V 



iV 



\A* 





Brass . 



3-4 



2-6 



•87 



•72 



Brass . . 



32 



1-7 



•73 



•63 



Copper . 



3-35 



2-45 



•86 



•57 



German silver 



30 



25 



•91 



•85 



It will be seen that in every case the rate of fall of 

 temperature along the thinner of the various pairs of rods 

 compared is more rapid than would be inferred according to 

 the usual formula from the distribution of temperature 

 observed along the thicker rod. Consequently this formula 

 cannot be legitimately used for the comparison of con- 

 ductivities, unless the radii of the rods compared are equal 

 and their surfaces in the same condition. 



The only way of escaping from this conclusion is by 

 supposing that the thermal conductivity of the smaller of 

 each pair of rods was less than that of the thicker one. 

 Although it seemed very unlikely that this should be so 

 in the case of every pair, it could not be regarded as im- 

 possible. To settle the matter, it was decided to determine 

 the absolute conductivity of at least one pair of rods. 



For this purpose Angstrom's method was adopted, in which 

 one end of the rod is alternately heated and cooled, and 

 the alternations of temperature observed at two points 

 along the rod. Fig. 1 is a diagram of the apparatus used. 

 C D is the rod, passing into a glass tube E, through which 

 cold water or steam can be passed as desired. E is held 

 up by a clamp, and F is a clamp supporting the rod at the 

 other end, no other clamps being used for the rod. S 1? S 2 , S 3 , 

 S 4 are brown-paper screens, about 2 ft. high, resting on the 

 table and open at the top ; S 3 is about 1 J ft. wide. A and B 

 are the two points whose temperatures have to be measured. 

 This was done by fine iron wires passing over the rod at the 

 required point, and held down by suitable weights. The wires 

 pass to a key K, through the galvanometer GJ-, to a vessel H 

 containing cold water. H contains the other junction, and a 

 thermometer T. A brass wire connects D to H. T 2 is a 

 thermometer reading the temperature of the air. Both ther- 

 mometers were divided into fifths and read by a telescope. 



As it was not possible to keep the temperature of the air 

 constant, allowance had to be made for the change. As the 



