HALL AND AYRES. — HEAT CONDUCTION IN IRON. 293 



Determination op the Difference op Temperature of the Ingo- 

 ing AND OuTCOMING WaTER AT THE ChAMBEK ABOVE THE DlSK. 



The method of making this measurement has already been indicated. 

 Two copper-German silver junctions, each like that shown in Figure 3, 

 were used, one at Jx in Figure 2, the other at J^ in the same figure. The 

 German silver wire used, about 0.015 cm. in diameter, was continuous 

 from /i, Figure'3, to the corresponding junction in the other plug. Its 

 length was perhaps 30 cm. The fine copper wire, 0.018 cm. in diameter 

 (from the same piece as the wires attached to the coatings of the disk), 

 leading from t/j, Figure 3, did not extend completely through the hard 

 rubber plug, or holder, but was soldered carefully, some distance from 

 the outer end of the plug, to a larger copper wire, which led off toward 

 an astatic galvanometer. The arrangement of copper wires at the other 

 plug was quite similar. The fine wires of each junction were coated 

 thinly with shellac. 



The two copper-German silver junctions thus described, or similar 

 ones,* were tested, or " calibrated," by means of streams of water, of a 

 known difference of temperature, flowing past the junctions according to 

 the arrows in Figure 3. The ditierence of temperature of the streams was 

 found by means of the same pair of thermometers that are indicated in 



* In accorrlance with my advice, I\Ir. A3Tes made 07ily such experiments in the 

 calibration of liis copper-German silver junctions as to show that their performance 

 differed but little from that of similar junctions used previously by myself. After 

 this, in all his calculations of the conductivity, he took his values of the thermo 

 e. m. f. of copper-German silver from Figure 8 of my previous paper, already re- 

 ferred to, "On the Conductivity of Mild Steel." In preparing the present paper, I 

 have had some misgivings as to the accuracy of these values, and therefore in 

 October, 1898, I made more experiments upon a pair of junctions quite similar to 

 those used by Mr. Ayres. Tlie results are given in the second column below. 

 The third column gives values, for the same temperatures, taken from the figure 

 used by Mr. Ayres : — 



Mean Temn Electromotive Force, in Volts, per I'' C. DifiFerence of 



Temperature of Junctions. 



20° 3 .00001743 .00001732 



37°.9 .00001826 .000018.30 



58°.2 .00001944 .00001942 



From the old observations and the new combined a curve representing the ther- 

 mo e. m. f. at temperatures ranging from 15° to 65° was constructed, and the values 

 of the conductivity found by Mr. Ayres were revised accordingly. The resulting 

 changes of conductivity were slight, but they had considerable effect upon the 

 estimated temperature coefficient of conductivity. — E. H. II. 



