OF THE NATIONAL PHYSICAL LABORATORY, 



363 



them through thin porcelain capillary tubes, and the two junctions of the platinum 

 with the copper forming the rest of the circuit were placed together in ice. 



The E.M.F. given by this element for a difference of 1 between its two hot 

 junctions is given in the table below.* This is obtained by direct comparison over 

 the whole range of a simple junction made up of wires from the same reels, which had 

 been similarly treated. 



One of the junctions was carefully placed and kept at the middle point of the gas 

 thermometer bulb, and the second was arranged so that it could be pushed backward 

 and forward into positions 2, 4, G, and 8 centims. to the right and left of this point, 

 observations being made of the differential E.M.F. produced in each of these 

 positions. 



From a number of such observations made after the different comparisons and 

 spaced over the interval 400 C. to ]()()0 C., curves were constructed showing the 

 distribution of temperature over this space for each of the furnaces used, and by 

 measuring these curves the difference between the average temperature of the bulb 

 and that of the central point, where the standard thermojunction was placed, could 

 easily be found. 



For the compensated furnace the mechanical centre was found not to quite coincide 

 with the position of highest temperature at the higher ranges, though over the lower 

 part of the scale the curves were practically symmetrical. 



The corrections obtained by this method from the different explorations were 

 plotted as ordinates against furnace temperatures as abscissae, and from the mean 

 curve thus obtained the following values were deduced as the mean corrections at 

 different temperatures : 



* In practice, when the junctions were placed as close together in the furnace as possible without actual 

 contact, owing to the inevitable small secondary effects arising from unsymmetric halting of the junction 

 wires, the E.M.F. round the circuit was not always zero. When, however, the junctions were so supported 

 as to nowhere touch the furnace wall, the total effect was always quite small and was allowed for in 

 each case. 



3 A 2 



