80 Intelligence and Miscellaneous Articles. 



ON HALL S PHENOMENON. BY M. AUG. RIGHI. 



Kesearches on this important phenomenon have led me to two 

 results, which I think may at once be usefully made known. 



The first result is in reference to the shape of the electrodes. 

 Instead of being in the form of a cross with four electrodes, it 

 may have any shape whatever, and has three electrodes. The cur- 

 rent enters or leaves by one of the electrodes, and leaves or enters 

 by the other two. The two partial currents circulate in an opposite 

 direction in the two circuits of each of the coils of a Wiedemann's 

 galvanometer; a suitable resistance introduced, or two other coils 

 traversed by the same total current, enable us to keep the needle 

 at zero, or nearly so. The instrument shows opposite variations 

 in the strength of the two partial currents as soon as we close the 

 circuit of the electromagnet, between the poles of which the plate is 

 placed. The direction of the deviation shows that the equipotential 

 lines are turned in a direction opposite that of the magnetizing 

 current for gold and the other metals in which Hall's phenomenon 

 has the same direction, and in the same direction as the magne- 

 tizing current for iron. By changing the direction of the current 

 which traverses the plate, we obtain galvanometric indications 

 which show that the phenomenon cannot be ascribed to a direct 

 action of magnetism on the current. All this will be explained in 

 a subsequent memoir. 



This mode of working enables us to obtain plates of very small 

 dimensions and irregular in shape, provided that they are thin 

 enough. 



The second result refers to Hall's phenomenon in bismuth, in 

 which metal it had not been previously studied, possibly owing to 

 the difficulty of getting it in plates sufficiently thin or sufficiently 

 large. By the preceding method I have discovered that Hall's 

 effect may be obtained with bismuth in the same manner as with 

 gold, but with extraordinary intensity, about 5000 times that of 

 gold. Thus, with the same strength of the current and the same 

 dimensions, a plate of bismuth of 0*079 millim. in thickness gave 

 deviations five or six times as great as a gold-leaf 0*000087 millim. 

 in thickness. Hall's effect may be obtained in bismuth even when 

 a simple steel bar is substituted for an electromagnet. 



I have lately succeeded in constructing very thin bars of bismuth 

 regular enough to give them the ordinary form of the cross. When 

 one or the other pole of a small bar-magnet is brought near the 

 plate, the Hall effect is obtained powerful enough to produce a 

 deviation of half a metre of the scale, and which is permanent if 

 the magnet is left in its place. 1 am working now at the produc- 

 tion of very thin plates of bismuth, and I am convinced that I 

 shall succeed in obtaining Hall's effect by the action of terrestrial 

 magnetism alone. — Journal de Physique, November 1883. 



