HALL. — CONDUCTIVITY OF MILD STEEL. 281 



in the water. "With the arrangement shown in Figure 5 the warmer 

 stream of water enters the upper chamber of the conduction apparatus, 

 and the cooler stream enters the lower chamber. This arrangement 

 was, however, used alternately with another, in which the water en- 

 tered S^ after leaving H^, the warmer stream going in this case to the 

 lower chamber of the conduction apparatus. One stream was usually 

 about 10° C. warmer than the other. 



It is now time to describe and discuss more fully the electrical de- 

 vices and apparatus used for measuring, 1st, the difference in tem- 

 perature of the two sides of the steel disk, and, 2d, the difference in 

 temperature of the incoming and outgoing water of the upper cham- 

 ber of the conduction apparatus. 



Calibration of the Copper-Steel Thermo-electric 

 Elements. 



It has been stated that there were thirteen copper wires leading off 

 from each copper coating of the steel disk. Each pair of wires, one 

 above and the corresponding one below, represents a copper-steel 

 thermo-electric element, consisting of a piece of steel about 0.3 cm. 

 thick between two pieces of copper. To calibrate these elements in 

 situ, that is, to determine by direct trial upon the disk the e. m. f. 

 corresponding to a given difference of temperature between the upper 

 and lower surfaces of the iron, is apparently impossible. Accordingly, 

 two slender bars, one about 1 square millimeter in cross section, the 

 other about one third as large in cross section, and each 7 or 8 cm. 

 long, were cut from the same plate of steel from which the disk had 

 been made, and tests were made with these,* copper wires having been 

 soldered to the ends. The apparatus used for these tests is shown in 

 Figure 6. J3^ and B2 in this figure are vertical brass tubes through 

 which streams of water at any desired temperature may be made to 

 run. T^j and T^ are Baudin thermometers, remarkably similar in size 



* Experiments, in wliich a thin bar of steel, of the same quality as those here 

 described, was subjected to longitudinal and to torsional stress, showed its 

 thermo-electric quality to be very little affected by such conditions. Fearing, 

 luiwever, lest the disk, which had been subjected to certnin processes which the 

 thin bars had not suffered, might differ from them in tlierrao-electric quality, I 

 have recently cut from the rim of a duplicate disk, made at the same time as 

 the other, two narrow strips, which I have compared directly with tlie thinner 

 of the two bars used in the copper-steel tests. Tliese strips proved to be so like 

 the bar in thermo-electric quality that it was difficult to make out with cer- 

 tainty any difference between them. 



