Related Properties of Crystalline Bismuth. 147 



The magnetic field was always at right angles to the plane 

 of t\\e plate, and therefore perpendicular to the crystallo- 

 graphic axis, while the current 3 was either parallel or 

 perpendicular to this axis. This current was led through 

 the plate by means of wires soldered to the copper strips 

 ab and cd. Two small brass screws # and h (fig. .1), which 

 pressed firmly on to the sides of the crystal, were used as 

 electrodes for measuring the Hall electromotive force E. g 

 could be moved in the direction of the length of the crystal, 

 and was adjusted so that when the current 3 passed the 

 electromotive force between g and h was as small as possible. 

 It was found impossible to adjust the contacts so that they 

 lay exactly on an equipotential line, but the error due to this 

 imperfect adjustment makes no perceptible difference in the 

 mean value of the constant. The electromotive force was 

 measured by the compensation method. By this method one is 

 independent of the change of resistance in the plate due to the 

 magnetic field. Before exciting the field a current of known 

 strength was sent through the plate, and the electromotive force 

 between the points g and It measured. The field was then 

 excited, the current 3 adjusted to the same value, and the 

 electromotive force between g and It again measured. The 

 difference between these two readings gave the Hall electro- 

 motive force B. The measurements were repeated with the 

 current reversed, and again, for both directions of current 

 with the magnetic field in the opposite direction. The mean 

 of the four values was taken as the measure of the effect. 

 In order to avoid heating of the plate due to the passage of 

 the current 3? the circuit was only closed momentarily. A 

 special key served to close this primary circuit, and im- 

 mediately afterwards the secondary circuit connecting the 

 Hall electrodes ; if the Hall electromotive force was not 

 exactly compensated then a deflexion of the galvanometer 

 was noted. By repeated trials two adjustments could be 

 found, for one of which a deflexion could be obtained in one 

 direction, and for the other in the opposite direction. These 

 adjustments differed from one another by not more than 1 per 

 cent., and the mean was taken as the balance-point. By 

 closing the primary circuit first, induction effects of the 

 primary on the secondary circuit were avoided. 



Tables III. and IV. contain the results for the two directions 

 and at the three temperatures, and the curves (rig. 5, p. 150) give 

 a graphic representation. The full-line curves correspond to 

 the case when the current 3 is parallel to the chief crystallo- 

 graphic axis, and the broken-line curves to the case when 3 

 is at right angles to this axis. 



L2 



