Magnetic Field on Thermal Conductivity. 531 



of 3550. This may be compared with the results quoted by 

 van Everdingen*. 



k = conductivity under zero field. 



k'= „ „ field applied. 





Field 3550. 



F = 1 ' ™' 



van Everdingen 



Leduc 



6000 

 7800 

 9000 



1-058 



1-057 



1-052 to 1-021. 



von Ettingshausen ... 



The above experiment was repeated with a bar o£ bismuth 

 obtained from Messrs. Johnson, Matthey & Co., and in this case 

 the von Ettingshausen and Nernst effect was found to be too 

 small for detection. In the previous case the change in 

 deflexion representing the effect was 10 scale-divisions ; but 

 although the conditions were all exactly the same in the trial 

 of the Johnson- Matthey bar no deflexion was observed on 

 applying the field. 



In order to measure the temperature-difference between 

 two points directly by a single reading a system of thermo- 

 junctions of german-silver and iron was arranged, the 

 junctions being insulated from the rod. These were thrust 

 into small radial holes in the rod, and kept out of contact by 

 thin slips of mica. No distinct effect of magnetic field on 

 the temperature-difference was detected by this method ; 

 moreover, it was found that the temperatures indicated by 

 these insulated junctions differed somewhat considerably from 

 those given by junctions soldered to the rod at the same 

 points. Therefore an arrangement in the form of a thermal 

 Wheatstone's bridge was adopted, whereby it was possible, 

 without admitting the Nernst effect, to have the junctions 

 soldered to the points between which the temperature-dif- 

 ference was to be observed. The two pairs of arms of the 

 bridge (fig. 1) were kept out of electrical contact by means 

 of small strips of insulation inserted between the metal pieces 

 and the walls of the slots in the copper blocks which were 

 soldered to the inside of the heating and cooling chambers. 

 The thermo-junctions of german-silver and iron attached to 

 the points A and B were connected in series through the 

 galvanometer, whose sensitiveness was 11 divisions deflexion 

 per 1° C. difference of temperature. The bars were wrapped 

 up in felt, and the magnet set so as to embrace between its 

 poles one of the arms of the bridge. Various forms of bridge 



* Leiden Communications, 25th March and 22nd April, 1898. 



