HALL. 



THERMAL AND ELECTRICAL EFFECTS IN SOFT IKON. 



49 



of the methods used in getting the data needed in making this estimate 

 may be desirable. 



On November 23, with boiling water in both boxes of the Thomson effect 

 apparatus from 12.45 o' clock on, observations made first with the couples 

 A' and il/(see Figure 3) joined in series, then with the junctions iVand P 

 joined in series, then with K and J/ once more, and so on alternately, 

 indicated gradients of temperature proportional, approximately, to the 

 numbers given below : 



Time 



3.11 

 3.24 

 3.33 

 4.10 

 4.21 

 4.29 

 4.58 

 5.08 

 5.15 

 5.23 

 5.31 



KM 



165 



151 

 134 



125 



i2n 



114 

 114 



N-P 



169 



129 



,116 



112 



109 



I, 



10 



It is evident that an approximately stable condition had been reached 

 during the last hour of the observations ; and the last three values found 

 from A-J/, with the last two found from N-P, were taken as indicative 

 of the temperature gradient in this stable condition. The average value 

 of this gradient at the four ends, as estimated from these observations, 

 was 0.104° per centimeter. A corresponding set of observations made 

 December 8 gave 0.108° per centimeter as the mean gradient at the four 

 ends. 



As the gradient at the middle of the bars must have been practically 

 zero, we cannot err greatly in taking the mean gradient from the ends to 

 the middle of each 20 cm. cylindrical part as 0.06° per centimeter, and the 

 temperature at the middle as 0.6° lower than that at the ends of this part. 

 This estimate is in fair agreement with the result of a rough direct meas- 

 urement of this difference made October 29. The mean temperature of 

 that part of bar a which lies between the mid point of A" A 7 and the mid 

 point of MM 1 must have been not far from 0.3° lower than the tempera- 

 ture of the water at the ends ; and a like estimate holds for bar /3. 



On December 8 an attempt was made to find the temperatures at various 

 points on the guard-ring bars, by means of thermo-electric couples X, T, 



VOL. XLI. — 4 



