602 PROCEEDINGS OF THE AMERICAN ACADEMY. 



couple is to indicate the gradient of temperature in its part of the bar 

 on which it lies. Cemented by asphaltum to the bar a, midway between 

 K and K' is a^, one junction of a copper-German-silver couple,, the other 

 junction of which is kept at a known temperature in a Crafts'* boiler 

 containing water or naphthalin, which apparatus is not shown in the 

 figure. The object of this couple is to indicate the mean temperature 

 of that part of a which lies between K and K'. The junctions a^, a^, a^, 

 tte, /3a, /3b, /3c, /3d, and (3^, have each a function similar to that of a^ ; for 

 each of them there is a corresponding junction in one or another of four 

 Crafts boilers. The bar Ge carries five copper- German-silver junctions, 

 6a, 6b, etc., and the bar Gie a like equipment, 16a, 16b, etc., correspond- 

 ing in position and function to the junctions a^, (3^, etc., already noted, 

 on the main bars. In every case the junction indicated in the figure is 

 connected with a mate, which can be kept at a definite temperature in 

 one of the Crafts boilers. Each junction attached to a guard-ring bar 

 is really lodged in a small hole drilled in the bar, insulation being 

 secured by means of thin-walled glass tubing, and fixity of position by 

 means of asphaltum varnish. Bars Gi and Gu, shown in Figure 2, are 

 similarly provided. We have, accordingly, on the main bars and the 

 guard-ring taken together, six a junctions, six b junctions, etc., each 

 set of six lying approximately, though by no means accurately, in one 

 cross-sectional plane. The effect of displacements from such a plane, 

 though for the most part eliminated in the end by the practice of 

 reversing the gradient of temperature in the apparatus, will require 

 notice in some cases. 



In every case the distance from a to b, from b to c, etc., was intended 

 to be 4 cm. The distance from pot to pot was very nearly 21 cm. 



The copper and German-silver wires, all about 0.02 cm. in diameter, 

 leading up from the various thermo-electric junctions which have been 

 described, were insulated, where insulation seemed necessary, by passing 

 them through short, thin-walled glass tubes. The wires from the main 

 bars ran almost straight up, as Figure 2 indicates, while those from the 

 guard-ring bars were made to follow the contour of the inner guard-ring 

 for a greater or less distance before leaving the packing with which the 

 guard-rings are surrounded. The wires from bar Gi, at the top, were 

 carried down to the bottom of the inner ring, as Figure 2 shows, and 

 then brought up on its outside. The object of these precautions was to 

 make sure that the gradient of temperature in the wires leading from 



* See a paper On Thermo-electric Heterogeneitij in Certain AIloi/s, especial};/ 

 German-Silver, by the autliors of the present paper, tliese Proceodiiiffs, March, 

 1900, for an account of tlie calibration of tiierrao-electric couples by use of the 

 Crafts apparatus. 



