HALL. — THERMAL AND ELECTRICAL EFFECTS IN SOFT IRON. 615 



ti,n = mean of temp's midway between the / and the m spirals. 



± U (( (( (( (( K .„ U I (( 



T,„, = diff. " " " " " I " m " 



T„^ = " " " " " " m " h 



A — strength of the main electric current in amperes. 



In finding the "general mean" for each column except the last 

 each quantity in the column is taken with a weight proportional to 

 the number placed on its level in the last column. 



The various ^'s and t's in this table were not all independently ob- 

 served. All those in the first part, East End Hot, are derived from 

 observations made February 5 to 7, with allowance for the change 

 which differences of barometric pressure would probably make in the 

 temperature of the main bars, the ends of which are in boiling water 

 or boiling naphthalin. 



The ^'s and t's for February 16-17 and March 2-3 are derived from 

 observations made February 23-24. The ^'s and t's for April 13-14 

 are derived from observations made April 14, during the same heating 

 that gave the A's for these days. 



In the derivation of the thermal conductivity values of iron which 

 are needed for use with the data given above, the temperature co- 

 efficient of this conductivity is taken as — 0.00U7 below 115°, and 

 as —0.0006 from this temperature up to 205°. We thus get approxi- 

 mately, hi being the conductivity at ti, etc. : 



0.1436 0.1396 0.1355 



The specific electric resistance of the iron, which also is needed in 

 the evaluation of the Thomson effect, was measured by Mr. Campbell 

 with the aid of the Crafts boilers, in the summer of 1905, and was 

 found to be 17,260 at 100° and 26,140 at 218.2°, with a mean temper- 

 ature coefficient 0.00661 (on the basis of the 0° value of the resist- 

 ance) between 100° and 218°. 



In the following table q^ indicates the first approximation to the 

 " Thomson effect heat " ; that is, the difference between the amount 

 of heat generated per second by the electric current between the / and 

 the h cross-sections of either bar and the amount accounted for by 

 Joule's law between the same cross-sections, </' being calculated by the 

 formula (4), p. 25, of our previous paper.^^ The value ^", the second 

 approximation to the quantity desired, obtained from q^ by taking 

 account of the difference of thermal conductivity due to the slight excess 



" These Proceedings, May, 1905. 



