HALL AND ATRES. 



HEAT CONDUCTION IN IRON. 



289 



Care was taken to make the thirteen circuits which could be, one at 

 a time, thus formed very nearly equal in resistance. It was possible to 

 use such copper connectors between the mercury wells as to throw all 

 the wires leading from the upper coating of the disk into multii)le arc 

 with each other, and all those leading from the lower coating into multiple 

 arc with each other, and to connect the two sets of wires in one circuit 

 with the galvanometer. The latter mode of connection was finally used 

 in the conductivity experiments ; but certain preliminary observations with 

 the single circuits were made in order to find whether the various pairs 

 of junctions on the disk were enougli alike in performance to justify con- 

 necting them in multiple. Tiie method of testing was to run a stream 

 of water at constant temperature through the apparatus on the under 

 side of the disk, and another stream at a different constant temperature 

 through on the upper side of the disk, and to note the galvanometer 

 deflections obtained from each of the pairs of junctions in turn. The 

 following table shows the result of the observations : — 



Deflections. 



Junctions. 



13 and 13' 



1 and 1' 



4 and 4' 



7 and V 



10 and 10' 



2 and 2' 



5 and 5' 



8 and 8' 



11 and 11' 



Oct. 23. 



4.91 



5.31 

 5.08 

 4.93 

 4.71 



5.06 

 5.03 

 4.76 

 4.66 



Oct 26. 



4.38 



4.70 

 4.70 

 4.47 

 4.33 



Oct. 26. 



4.46 



4.90 4.87 



4.78 4.70 



4.55 4.56 



4.53 4.40 



4.51 

 4.63 

 4.40 

 4.30 



Nov 3. 



4.86 



5.25 

 5.20 

 4.80 

 4.75 



5 15 

 5.06 

 4.75 



4.76 



Nov. 3. 



4.74 



5.19 

 5.00 

 4.60 

 4.55 



5.05 



5.00 

 4.68 

 4.68 



Mean. 



4.67 



5.10] 



4.95 ' 



4.69 

 4.59 



)>4.83 



4.89] 

 4-88 I ^„ 

 4.61,^^-^^ 

 4.55 I 



3 and 3' 



6 and 6' 



9 and 9' 



12 and 12' 



4.90 

 4.70 

 4.66 



4.60 



4.67 

 4.67 

 4.40 

 4.33 



4.59 

 4.59 

 4.40 

 4.35 



5.02 

 4.99 

 4.76 

 4.65 



4.98 

 4.98 

 4.70 

 4.70 



4.83 ] 

 4.79 ' 

 4.58 

 4.53 j 



}^4.68 



An examination of this table, in connection with Figure A, leads to the 

 conclusion that the mean difference of temperature between the two sides 

 of the disk increases from the centre to the circumference about 3 per 

 cent. It appears, too, that the mean difference of temperature between 

 top and bottom is greater along the radius 1-2-3-13 than along the 



VOL XXXIV. — 19 



