hall. — thermal and electrical effects in soft iron. 613 



Measurement of the Thomson Effect. 



Our general method of procedure in getting data on the Thomson 

 effect itself, and our method of handling these data, were so fully set 

 forth in the paper ^ already mentioned that, inasmuch as we have made 

 no fundamental changes in these methods, we shall not describe them 

 in detail here ; but certain modifications may well be mentioned. In 

 the previous paper occurs the following sentence : " It will be ob- 

 served that such a day's work includes six complete cycles, or runs, 

 through all the pairs of corresponding couples from M P to K N and 

 back, three runs with the main current in condition F [cold to hot in 

 the front bar, ^], and three with this current in condition B [cold to 

 hot in the hack bar, a]." In the work described in the present paper 

 we have usually, though not always, made more cycles during a single 

 heating, as the following practically complete table shows : 



Date. Cycles. Date. Cycles, 



B F B F 



January 31 (2 2} 4 March 2-3 U ^ 1 10 



February 5-6 -^ 3 6 V 12 ^ (5 



16-17 



April 13-14 jo ^ |- li 



18 



17-18 \1 8 M7 



The main data obtained from the experiments just indicated, 

 and from supplementary observations made at other times on the 

 temperature differences of the main bars, are exhibited in the 

 table on p. 608, in which 



Aj =1 the difference of temp, gradient at low temp, spirals. 

 A,„ = " " " " " " middle " 



A^ = " " " " " high 



The + sign indicates that the steeper gradient is in the warmer bar, 

 and vice versa. 



ti = the mean of the temperatures at the low temp, spirals. 



" middle " 

 " '' «« " high " " 



diff. " " " low 



" middle " 

 " " high " 



as the value of this coefficient —0.00065 in one bar of iron and —0.00014 in an- 

 other bar of iron, between 18° C. and 100° C. 

 • These Proceedings, May, 1905. 



