276 Mr. A. W. Porter on the Influence of the 



although only one sphere was employed. I subjoin a 

 table : — 



From c. 



Peclet's results from spheres '000417. 



MncFarlaue's and Bottomley's results 

 in large enclosure, assuming 

 Bottom ley's value for radiation ... 



Bottomley's results for sooted sphere 

 in small enclosure 



\ about -0004. 



■000192 for excess of 85° C. 

 181 „ „ 65° C. 

 168 „ „ 45° C. 

 160 „ „ 35° C. 



Bottomley's results for silvered and 



highly polished sphere in small j. -000108 for excess of 65° 0. 

 enclosure 



Place against these the values obtained from cylinders : — 



From c. 



Peclet's results in large enclosure ... '000508 for 0° excess. 



Ayrton and Kilgour's, in enclosure 

 of 2'54 centim. radius 



•000160 with wire at 300° 0. 

 143 „ „ 200° O. 

 139 „ „ 150° C. 

 126 „ „ 100° C. 



In order to throw light on this question, I have started 

 experiments, in conjunction with Mr. Eumorfopoulos, on 

 cylindrical rods in cylindrical enclosures of different radii. 

 Experiments made so far are as follows : — 



A brass rod '483 centim. radius has soldered on it two 

 thermoelectric junctions of iron and german-silver at a dis- 

 tance apart of 10 centim., each of which is part of a couple 

 whose other junction is kept cold in a water-pot. The rod is 

 heated at one end by steam until the steady state of tempera- 

 ture is attained, and the ratio of the temperatures (reckoning 

 from enclosure temperature as zero) of the two junctions is 

 measured successively with different water-jackets embracing 

 the rod. 



The general arrangement is shown in fig. 2 (p. 277). 

 Tap- water is passed through the water-jacket at such a rate 

 that its temperature as it enters is sensibly the same as when 

 it leaves the jacket. This temperature is read by a ther- 

 mometer, as also are the temperatures of the cold junctions. 

 Then either the junction at A and its corresponding cold junc- 

 tion, or the junction at B and its corresponding cold junction, 

 can be connected to a potentiometer, and the thermoelectric 

 E.M.F. in each case determined. 



The potentiometer was standardized from time to time by a 

 Clark's cell which is so connected (switches not shown) that 

 the same galvanometer does for all three operations. 



