336 Dr. Tyndall on the Polarity of Bimuth, 



librium for the bar was at an angle of 85°, or thereabouts, with 

 the axial line. 



7. Looking from the south pole towards the north pole of the 

 magnet, or in the direction of the current as it passed over the 

 bar, that end of the bar which faced the south pole swung to the 

 left: 



8. The current through the helix being interrupted and the 

 bar brought once more to rest in the axial position (which of 

 course is greatly facilitated by the proper opening and closing of 

 the circuit), a current was sent through in the opposite direc- 

 tion, that is from the north pole to the south ; the end of the 

 bar, which in the former experiment was deflected to the left, 

 was now deflected an equal quantity to the right. I have re- 

 peated this experiment a great number of times and on many 

 difterent days with the same result. 



9. In this case the direction of the current by which the 

 magnet was excited was constant, that passing through the helix 

 which surrounded the bismuth cylinder being variable. The 

 same plijenomena are exhibited if we preserve the latter constant 

 and reverse the former. 



10. A polar action seems undoubtedly to be indicated here ; 

 but if a polarity be inferred, it must be assumed that the north 

 pole of the magnet excites a south pole in the bismuth, and the 

 south pole of the magnet a north pole in the bismuth ; for by 

 reference to the direction of the current and the concomitant 

 deflection, it will be seen that the deportment of the bismuth is 

 exactly the same as that which a magnetized needle freely 

 suspended between the poles must exhibit under the same 

 circumstances. 



11. The bar of bismuth was then removed, and a little bar of 

 magnetic shale was suspended in its stead ; it set axial. On 

 sending a current through the surrounding helix, it was de- 

 flected in the same manner as the bismuth. The 'piece of shale 

 was then removed and a little bar of iron was suspended within 

 the helix ; the residual magnetism which remained in the cores 

 after the cessation of the exciting current was sufficient to set 

 the bar axial ; a veiy feeble current was sent through the helix 

 and the deflection obsenxd, — it was exactly the same as that of 

 the bismuth and the shale. 



12. These results being different from those obtained by 

 M. Poggendoi'ff', I repeated his experiment with all possible care. 

 A bar of ordinary bismuth, an inch in length and about 0*2 of 

 an inch in diameter, was suspended within the helix ; on exciting 

 the magnet, it receded to the equator and became finally steady 

 there, llie axis of the bar thus coincided with the axis of the 

 helix. A current being sent through the latter, the bar wai 

 distinctly deflected. Supposing an observer to stand before the 



