62 Lloyd — Thermo-magnetic and Galvano-magnetic 



plate is at the higher potential. This direction is the same as 

 Lownds* has found in bismuth at ordinary temperatures. 



Two suggestions have been madef which might explain the 

 origin of the longitudinal effects. One is, that the thermo- 

 electric-power is altered in a magnetic field. Lownds has 

 demonstrated that the effect observed by him cannot be so 

 explained. The other suggestion is, that owing to a change 

 in heat-conductivity, the temperatures at the junctions are 

 changed, thus changing the E.M.F. in circuit. This explana- 

 tion is plausible where the effect was measured by compen- 

 sating the E.M.F. in zero field, and observing the change 

 produced by the field. Lownds does not mention whether he 

 made any test of this possibility, but in one of his experiments 

 at least (p. 779) it is evident that the E.M.F. could not have 

 had this origin. 



The longitudinal effect has been observed in bismuth also by 

 van Everdingen4 and found to vary with the strength of field 

 according to the same law as the variation of resistance. 



In my own experiments the temperatures at the junctions 

 were observed in the given field as well as in zero-field, and 

 allowance made for the change. Theory§ indicates that longi- 

 tudinal effects should be proportional to the square of the 

 strength of field, and independent of its direction. I have 

 noticed that in every case the effect was greater for one direc- 

 tion than for the other. The mean value for a field strength 

 of 2750 gausses, mean temp. = 39°, difference of temp. = 19°, 

 was 1050 microvolts. 



If we regard the transversal differences of temperature and 

 potential as due to the transfer of heat and electricity respect- 

 ively across the plate, we notice that in tellurium the transfer 

 of electricity is always in the same direction as the transfer of 

 heat. That is, the edge which is raised in potential is also 

 raised in temperature. Moreover, to produce the same effects 

 with a current of electricity as with a current of heat, the 

 direction of flow must be the same for both. For example, 

 the edge which is warmed by the Leduc effect has its potential 

 increased by the simultaneous Nernst effect. And to warm 

 the same edge by the Ettingshausen effect as by the Leduc 

 effect, the primary currents of electricity and heat must have 

 the same direction. 



In bismuth, the reverse is the case, the heat and electricity 

 being transferred in opposite directions. Thus the edge which 

 is warmed by the Leduc effect has its potential lowered by the 

 simultaneous Nernst effect. And to warm that same edge by 



* Ann. d. Phys., iv, p. 776, 1901. 



\ Grimaldi, Nuov. Cim. (3), xxii, p. 5, 1887; Goldhammer, I c. 



% Leiden Comm., No. 48, 1899. §Drude, Ann. d. Physik, iii, p. 377, 1900. 



