332 Journal of Agricultural Research voi. v.no.8 



to end and joined with silver solder. The wires were then bent at the 

 junctions into a grid, with the parallel lines about 5 mm. apart and with 

 copper and constantan alternating. Each constantan wire and three of 

 the five copper wires are about 7 cm. long, so that the distance between 

 the two opposing sets of junctions is the same as that between the copper 

 wall and the zinc wall. The two other copper wires, which are at opposite 

 ends of the series, are longer, to form leads for the element, as explained 

 below. 



Wire of the size stated was used chiefly because it was m.ost readily 

 available and seemed quite well adapted to the type of element con- 

 structed. Theoretically, a small wire would be preferable, because of 

 smaller thermal conductance, but the support in which each element is 

 mounted probably greatly delays change in temiperature of the wires 

 between the junctions, while affording opportunity for rapid change at 

 the junctions. This support consists of a hard maple rod or spindle 

 about 10 cm. in length and 15 mm. in diameter. A recess 8 mm. wide 

 and 2 mm. deep is cut around the spindle 3 cm. from one end, and in the 

 surface are 10 equally spaced longitudinal slots, each nearly i mm. wide 

 and 2 mm. deep. The five copper and four constantan wires which, 

 ioined alternately in series in a grid, as described above, comprise the 

 four differential thermocouples of an element, were forced into these 

 slots until they were about a millimeter below the surface of the wood 

 and to that extent were protected against contact with the metal sleeve 

 and thimble by which the thermoelement is supported in the walls, as 

 explained below. By means of a cut between two adjoining slots near 

 the center of the spindle the copper wire at one end of the series is doubled 

 back and extends parallel with the copper wire at the other end of the 

 series, the two projecting from one end of the spindle and providing 

 terminals for the element. The spindles with the wires thus embedded 

 were boiled in paraffin for two or three hours, so that they would not 

 swell or shrink with changes in the humidity of the air. 



The temperature of the wires thus embedded in the spindle is probably 

 that of the spindle and therefore changes slowly — i. e., the temperature 

 gradient in each wire is quite Uke that of the others in the element and 

 is relatively constant for considerable periods. On the other hand, the 

 junctions between the copper and the constantan wires are not embedded, 

 one series of four alternate junctions projecting into the air at one end 

 of the spindle, while the series of opposing junctions projects into the air in 

 the recess near the other end of the spindle, so that changes in the tempera- 

 ture of theuir surrounding them affect the junctions quickly. 



To keep each element in place between the two metal walls a short 

 copper sleeve is passed through a hole in the zinc wall, the sleeve being 

 soldered to the zinc at the edge of the hole to insure good thermal con- 

 ductance; and directly opposite, with its open end facing that of the 

 sleeve, a short copper thimble is firmly soldered to the outer surface of 



