HALL. — THERMO-ELECTRIC HETEROGENEITY IN ALLOYS. 565 



when the data for this curve were taken may have been as high as 20°, we 

 will take 500 as the ratio of the electromotive force of a (copper)-(german- 

 silver) couple to that of the most effective (german-silver)-(german- 

 silver) couple which could be made from contiguous parts of the wire which 

 gave the curve C 5 , the temperatures to be the same in both couples. 



When one junction of this (copper)-(german-silver) couple was in con- 

 tact with an ice surface about 1 cm. wide, so that contact between metal 

 and ice extended about 0.5 cm. in each direction from the junction, the 

 electromotive force developed was about 3 per cent greater than when 

 a sharp edge only of ice was used, which may be taken as indicating 

 that the sharp edge cools the junction to the neighborhood of 0°.5. 



When the german-silver wire was touched by a sharp edge of ice at 

 a distance of 1 cm. from a junction, the electromotive force was such as 

 to indicate that the junction itself was thereby cooled about 3 degrees. 

 Of course the junction, having the highly conducting copper on one side, 

 was cooled somewhat less in this case than a similarly placed point in a 

 continuous german-silver wire would have been. When the copper wire 

 was touched by the. edge of ice at a point distant 1 cm. from the junc- 

 tion, the junction was apparently cooled about 12 degrees. 



It appears, then, that in our experiments along a straight piece of 

 german-silver wire, exposed to the air of a room at 18° C, the point 

 in contact with the sharp edge of ice was near the temperature 0°.5, 

 while the points a centimeter distant from the point of contact were, 

 perhaps, near the temperature 13°, the mean temperature gradient for 

 the first centimeter in each direction from the point of contact being, 

 therefore, let us say, 12°. 



We may, for present purposes, safely regard the temperature change 

 caused by the edge of ice as insensible at 2 cm. from the point of 

 contact. The thermo-electromotive force caused by the inequality of 

 temperature due to the ice is, therefore, dependent on the difference 

 of thermo-electric quality of two pieces of wire, each 2 cm. long, 

 which extend in opposite directions from the point of contact with the 

 ice, and in each of which there is a temperature range of about 18°. 

 Let us call these pieces section I and section r, respectively. The 

 thermo-electromotive force under discussion is equal to the change of 

 electromotive force which would result in one of our ordinary (copper)- 

 (german-silver) couples, if we were to cut out a piece of german-silver 

 having the quality of section I and substitute for it a piece having the 

 quality of section r, the temperatures at the two points of cutting and 

 joining being, approximately, 0° C. and 18° C, respectively. 



