HALL. THERMAL AND ELECTRICAL EFFECTS IN SOFT IRON. 33 



no differences of gradient in the opposing couples. Applying to the 

 mean A's given ahove the corrections made necessary by this considera- 

 tion, we get, as the improved A's, 



A, A m A, 



+ 0.0337 + 0.0040 — 0.0374 



Dividing each of these quantities by 1.00*2, which according to a pre- 

 vious statement may be taken as the average distance between the two 

 junctions of a single couple, we get 



A, A m A h 



0.03363 0.00400 — 0.03733 



The mean temperatures found for the mid points of the couples K K' 

 N N', etc., by use of the juuetions E, H, etc., are 



13.0° 51.1° 90.3° 



According to a previous investigation,* the value of the thermal con- 

 ductivity of the iron here studied is 0.1528 at 28° C, with a mean tem- 

 perature coefficient — 0.0003 for the range from 28° to 58°. A very 

 recent investigation (see p. 52), made with the same apparatus that has 

 been described in this paper, gives for the mean temperature coefficient of 

 thermal conductivity from 13° C. to 87° C. the value — 0.0007. Using 

 this later value and taking k as 0.1528 at 28°, we get 



0.1543 0.1493 0.1461 



From the difference of temperature of the two bars, as already given, at 

 the sections c, m, and h respectively, we can make the somewhat uncertain 

 estimate that the difference ( t a — t b ), occurring in equation (4), is, between 

 the sections c and h, 0.21°. 



Recent measurements, by Mr. Campbell, of the electrical conductivity 

 of the iron here used give for K, in equation (4), the value 11365 X 10~ 9 

 ohms and show that y is approximately 0.005. 



Putting, in equation (4), 



* Physical Keview, May-June, 1900, p. 305. 



VOL. XLI. — 3 



