60 Lloyd — Therm o-magnetic and Galvano-magnetic 



positive. It is in the opposite direction in bismuth. To 

 determine the Hall effect, the difference of potential was meas- 

 ured between the copper wires connected to opposite edges of 

 the plate. This P.D. includes the thermo-effect due to the 

 two points of contact not being at the same temperature. If 

 the temperature remained constant, the thermo-effect could be 

 eliminated bj taking readings for both directions of the field. 

 But the temperatures are not constant, the Ettingshausen effect 

 producing a change whenever the field is reversed, and this 

 must be allowed for. It was found in this case to affect the 

 second significant figure by one unit only and hence is 

 neglected. The coefficient of the Hall effect, E, is defined by 

 the equation 



E=RH C . 

 t 



In a field of strength 4,000 gausses, the value of R found 

 was 430 C.G-.S. at a temperature of 65°. 



Change of resistance — The resistance was determined by the 

 difference of potential of two points between which a current 

 was flowing. The current and P.D. were measured with 

 Weston ammeter and voltmeter. The accuracy of the observa- 

 tions would serve to determine a change in resistance of one- 

 half of one per cent. No change was found in fields up to 

 5,500 gausses. Other observations* on tellurium show an 

 increase in resistance less than this. 



Ledue effect. — If the heat flow be from right to left and the lines 

 of force are directed away from the observer, the upper edge 

 of the plate is cooled. The direction in bismuth is the reverse 

 of this. The coefficient of the Led tic effect, S, is defined by 

 the equation 



T = S6H^ T 

 d\ 



dr 

 where b is the breadth of the plate and -r the longitudinal 



temperature-gradient. Values of S between 0-000002 and 

 0*000005 were obtained at temperatures between 30° and 38° 

 in fields of strengths from 2,500 to 5,200 gausses. The 

 coefficient appears to increase with the temperature. 



Nernst effect. — If the heat flow be from right to left, and the 

 lines of magnetic force are directed away from the observer, 

 the upper edge is at lower potential. In bismuth the direction 

 is the same. The coefficient of Nernst effect, Q, is defined by 

 the equation 



E-Q6H* 



* Goldhammer, Wied. Ann., xxxi, p. 360, 1887. 



