Thermoelectric Properties of Crystalline Bismuth. 329 



two small copper screws e and/ (fig. 4) 6 mm. apart, passing 

 through a plate of ebonite L which fits exactly between the 

 coppers D and E. The electrodes were made to press firmly 

 on the plate by means of two brass screws g and h (fig. 4), 

 passing through the ebonite into the wooden base H. A 

 third thermoelement k of iron-constantan was also pressed on 

 the centre of the plate by means of this ebonite piece. This 

 couple was made by drilling a hole through the centre of a 

 small iron screw, which was carried in the centre of the 

 ebonite plate, and passing through this hole a vulcanite- 

 covered constantan wire, the constantan wire being soldered 

 to the iron at the extreme point of the screw. The three 

 thermo-elements d, k, and d' were in the same vertical straight 

 line, the distances between d and k, and k and d! being each 

 6*5 mm. The line joining the electrodes e and /was exactly 

 midway between the thermo-elements at d and k. 



The whole was carefully packed round with cotton-wool 

 and placed between the poles of the electromagnet, so that 

 the lines of force cut the plate at right angles. 



The electromotive forces were measured by the ordinary 

 compensation method. 



The field-strengths were measured by the observed change 

 of resistance of a previously calibrated bismuth spiral. 



§ 5. Tlie Longitudinal Effect. — The longitudinal effect was 

 calculated by means of the formula used in my former paper * 



where p is the electromotive force excited by the field between 

 two points whose temperatures are t Y and t 2 respectively. 

 p is reckoned positive or negative according as it is opposed 

 to, or in the direction of, the current of heat. The electro- 

 motive force between the two points was measured (1) with- 

 out field e v (2) with field in one direction e 2 , (3) with field in 

 the opposite direction e s , (4) without field e 4 . The absolute 

 value of n is then (e 2 + e 3 ) - fa + g,) 



2(* 3 -*i) 



the electromotive forces being expressed in microvolts and the 

 temperatures in degrees centigrade. 



Case 1, § 3. The chief crystallographic axis is parallel to 

 the current of heat and the magnetic lines of force per- 

 pendicular to the plane of the plate. The results for mean 

 temperatures 53°'3, 4°*8, — 36°'5, and —94° are given in 

 Table L, and plotted as the full-line curves in fig. 5. The 

 field-strengths H are abscissas, the corresponding values of n 

 ordinates. 



* Louis Lownds, Ann. d. Physik, iv. p. 776 (1901). 



