HALL. — THERMAL AND ELECTRICAL EFFECTS IN SOFT IRON. 599 



steepness from 50° C. to tlie neighborhood of 130° C. ; though Lecher 

 is in disagreement with others as to the general change of inclination 

 of the line above this temperature. 



It is noted that lead, in which the Thomson effect is undoubtedly very 

 small at certain temperatures, has apparently been carefully examined 

 only in the neighborhood of 50° C. (by Le Roux, by Haga, and by Bat- 

 telli) and 110° C. (by Battelli), and the question is raised whether it is 

 not desirable to examine copper, which is mechanically a much better 

 material to work with than lead, very carefully through the widest 

 practicable range of temperature, with a view to establishing a reference 

 line for the thermo-electric diagram. 



Incidentally, it is observed that the thermal conductivity of calcined 

 oxide of magnesium, packed to a density of about 0.17 gram per cu. cm., 

 is considerably less, perhaps 30 percent less, at 218° C. than at 100° C. 



General Arrangement of Apparatus. 



The general method followed in our recent work was the same as 

 that which was described in the paper already referred to ; but, as many 

 changes in detail were necessitated by working at a higher tempera- 

 ture, it seems desirable to show by means of diagrams the later form of 

 the apparatus. 



Figure 1 shows a horizontal section through the middle at the level 

 of the two main iron bars, a and y8, each 1 cm. in diameter, which were 

 the subjects of the research. The pots are of cast iron, about 16.5 cm. 

 long, in the direction of the bars, 27 cm. wide, and 25 cm. deep. 



Around the main bars are two " guard-rings " (see Figure 2, a verti- 

 cal cross-section of the apparatus), each consisting of twenty iron bars 

 about 0.6 cm. in diameter, the primary object of which is to lessen 

 lateral outflow of heat from the main bars. The outer guard-ring is a 

 new feature of the apparatus. 



Behind the brass nuts n on the main bars are, first, washers of brass ; 

 next, washers of leather prepared for plumbers' use, to bear a rather 

 high temperature.3 Mica and a plumber's cement, consisting of silicate 



' A trial which we now know to have been too short had persuaded us tliat 

 this prepared leather would withstand the action of hot naphthalin. When, after 

 a long course of experiments, the main apparatus here described was closely 

 examined, it appeared that the leather washers had been reduced to dust. But 

 the cement had apparently held, so that, so far as we know, there was no leakage 

 at the flanges of the main bars during the experiments on the Thomson effect, 

 though there was such leakage in experiments made later, in July, 1906. See 

 p. 619. 



