Related Properties of Crystalline Bismuth. 143 



between any two points on the plate. These contacts were 

 adjusted so as to touch two points lying on a line at right 

 angles to the copper strips. A current of known value was 

 then passed through the plate, and the difference of potential 

 between these two points was measured. The field was then 

 excited and the determination repeated, the current through 

 the plate being adjusted to the same value as before. If e Q 

 is the difference of potential without the field, w the resist- 

 ance between the points, and I the current strength, e and w 

 the corresponding values with the field, then 



w to 

 iv _ e 

 iv e 

 .'. Percentage Increase ) in .w-H) n ir\n e ~ e o 

 of Resistance in Field J = 10U ~^~ = 1U0 ^ ' 



The current was only made momentarily, so that heating- 

 effects were avoided. The electromotive force was measured 

 by the compensation method, a D'Arsonval galvanometer 

 being used of resistance 1000 ohms. Induction effects from 

 the current used to produce the fall of potential were avoided 

 by using a key which first made this current, and afterwards 

 closed the galvanometer circuit. The magnetic field being- 

 completed the whole time, while observations were being- 

 made with field, had no inductive influence. The current I, 

 which was of the order 1 to 2 amperes, was measured by an 

 amperemeter reading to ^ per cent. The electromotive forces 

 were of the order 300 to 600 microvolts and could be deter- 

 mined to at least 1 per cent. 



The observations were made : — 



(1) At ordinary temperature, the whole plate being 

 immersed in a petroleum bath. 



(2) At —79° C. in a mixture of solid carbon dioxide and 

 ether, contained in a silvered Dewar vessel. 



(3) At - 186° C. in liquid air. 



The absolute resistance between the two electrodes e and/'. 



viz., iv = j enables us to calculate approximately the 



specific resistance p of the plate. 

 We have w % a 



where a is the cross-section of the plate in square centimetres, 

 and I the distance in centimetres between the electrodes. 

 Since I was of the order 5 mms. and slight irregularities 

 existed on the edges of the crystal, this result can only be 

 regarded as approximate. The determination of p for any 



