including an Examination of the Magnetic Field. 335 



an electric current through the latter the bar was weakly de- 

 flected in a certain direction, and on reversing the current, a 

 feeble deflection in the opposite direction was observed. The 

 deflection was such as must follow from the supposition, that 

 the north pole of the magnet had excited a north pole in the 

 bismuth, and the south pole of the magnet a south pole. 



4. It will be at once seen that a considerable mechanical dis- 

 advantage was connected with the fact that the distance from 

 pole to pole of the transverse magnet was very short, being 

 merely the diameter of the bar. If a piece of bismuth, instead 

 of setting equatorial, could be caused to set axial, a mechanical 

 couple of far greater power would be presented to the action of 

 the surrounding current. Now it is well known that bismuth 

 sets in the magnetic field with the plane of most eminent clea- 

 vage equatorial ; hence the possibility, that if a bar of bismuth 

 could be obtained with the said plane of cleavage perpendicular 

 to its length, the directive power of such a bar might be sufii- 

 cient to overcome the tendency of its ends to proceed from 

 stronger to weaker places of magnetic action and to set the bar 

 axial. After repeated trials of melting and cooling in the labo- 

 ratory of Professor Magnus in Berlin, I succeeded in obtaining a 

 plate of this metal in which the plane of most eminent cleavage 

 was perpendicular to the flat surface of the plate, and perfectly 

 parallel to itself throughout. From this plate a little cylinder, 

 an inch long and 0'2 of an inch in diameter, was cut, which being 

 suspended horizontally between the excited poles turned strongly 

 into the axial position, thus deporting itself to all appearance as 

 a bar of iron. 



5. About 100 feet of copper wire overspun with silk were 

 wound into a helix of such a dimension that the cylinder was 

 able to swing freely within it ; through a little gap in the side of 

 the helix a fine silk fibre descended, to which the bar was attached ; 

 to prevent the action of the bar from being disturbed by casual 

 contact with the little fibrous ends protruding from the silk, a 

 coating of thin paper was gummed to the interior. 



6. The helix was placed between the flat poles of an electro- 

 magnet, so that the direction of its coils was from pole to pole. 

 It being first ascertained that the bar moved without impedi- 

 ment, and that it hung perfectly horizontal, the magnet was ex- 

 cited by two of Bunsen^s cells ; the bar was immediately pulled 

 into the axial line, being in this position parallel to the sur- 

 rounding coils. A current from a battery of six cells was sent 

 through the helix, so that the direction of the current, in the 

 upper half of the helix , was from the south pole to the north pole 

 of the magnet. The cylinder, which an instant before was 

 motionless, was deflected, forming at the limit of its swing an 

 angle of 70° with its former position ; the final position of equi- 



2A2 



