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



the cross- sections a and e, a distance of 16 cm., exceeds the mean 

 temperature of the inner guard-ring bars between the same two planes 

 approximately 1.48°. 



The mean temperature gradient, leading toward the middle, at the 

 points a and e of the main bars was found, according to the evidence 

 of (3) above, to be approximately ().()(5G° per cm. The diameter 

 of each bar being 1 cm., and its thermal conductivity at 100° ap- 

 proximately 0.145, according to our observations of last year, we find 

 as the amount of heat led into each bar per second, by conduction past 

 a and e, 



2 X T X 0.066 X 0.145 = 0.0150 calorie. 

 4 



This is also, when a stable condition is reached, the amount of heat 

 lost from each bar per second by lateral flow from the parts between 

 a and e. 



As the lateral outflow per second from each bar per 1 degree excess of 

 mean temperature of the bar above the mean temperature of the inner 

 guard -ring, all relating to the region between the cross-sections a and 

 e, we have, then, 



0.0150 ^ 1.48 = 0.0101 calorie. 



On February 8 and 9 the lateral flow was measured again, all the 

 pots in use now containing naphthalin boiling under atmospheric 

 pressure, which was about 77.3 cm. The boiling temperature of high- 

 grade naphthalin, such as was used in the Crafts boilers, is 218.8° at 

 this pressure. That of the very cheap variety, which was used in the 

 square pots at the ends of the bars, is not so well determined ; but 

 there was no need of an accurate knowledge of the temperature in 

 these pots. 



Naphthalin was boiling in all of the pots about 7 P. M. on February 8, 

 and continued in that condition until after 6 a. m. of February 9, opera- 

 tions being continued through the night. The first thermo-electric ob- 

 servations recorded were made about 3 a. m., and the last about 6.20 a. m. 

 The following statement will give some indication of the degree of con- 

 stancy of temperature which was maintained during these observations. 

 Under Sensitiveness are given the galvanometer deflections produced by 

 a current of a certain constant strength used in determining the sensi- 

 tiveness of the instrument ; under Spirals, the deflections given by the 

 couples K K', M M', N N', P P', joined in series ; under M against G, 

 the deflections given by the main bar a's and e's joined in series, in 

 opposition to the guard-ring a's in series. 



