340 



Dr. Tyadall on ilie Polarity of Bismuth, 



Fig. 3. 



ilH 



of the h^ix (fig. 3). Oil exciting tbc 

 maguet, the beam came to rest in tlie 

 position of the hard hne ; on sending a 

 cun-ent through the hehx in the direc- 

 tion of the arrow, the sphere loosed, 

 moved towards the north pole, and 

 came to rest in tlie dotted position. 

 If while in this position either the 

 current of the magnet or the current of the helix was reversed, 

 the sphere went back; if both were reversed simultaneously, the 

 sphere stood still. 



19. From these facts we learn, that if the magnetic field be 

 dividedintofourcompartments, asinfig. 4, Y\£c 4 



the passage of an electric current through a 

 helix placed therein, the direction of the cur- 

 rent in the upper half of the helixbeingthat 

 indicatedby the arrow, will weaken the force 

 in the first and third quadrants, but will 

 strengthen it in the second and fourth. With the aid of this 

 simple fact we can solve every experiment made with the bismuth 

 bars. In (12.), for instance, it was found that when an observer 

 stood before the magnet with a north pole to his right and a 

 South pole to his left, a current passing through the upper half 

 of the helix from the north to the south pole deflected a bar of 

 ordinary bisnmth, which had previously stood equatorial, so that 

 the end presented to the observer moved towards the north pole. 

 This deportment might be inferred from the constitution of the 

 magnetic field; the bar places its ends in quadrants 1 and 3, 

 that is, in the positions of weakest force. 



20. The experiments (7, 8.) with the other bar are capable of 

 an explanation just as easy. Preserving the arrangement as in 

 the last figure, the bismuth bar, which previously stood axial, 

 would be deflected by the surrounding current, so that its two 

 ends would occupy the quadrants 2 and 4, that is, the positions 

 of strongest force. Now this is exactly what they did in the 

 magnetic field before the passage of any current, for the bar set 

 axial. It was first proved by Mr. Faraday, that the mass of a 

 bismuth ciystal was most strongly repelled when the repulsive 

 force acted parallel to the planes of most eminent cleavage ; and 

 in the magnetic field the superior repulsion of these planes causes 

 them always to take up that position where the force is a mi- 

 nimum. It is the equatorial setting of these planes which 

 causes the bar at present under consideration to set axial. The 

 planes of cleavage being thus the true indicators, we see that 

 when these set from the first to the third quadrant, or in the line 

 of weakest action, the ends of the bar must necessarily occupy 

 the second and fourth, which is the deportment obseiTcd. 



