Bodies are repelled from the Poles of a Magnet. 269 



will now consider whether a bar of Insmuth exhibits similar 

 effects. 



Fig. 39 Plate IV. represents the disposition of the apparatus 

 used in the examination of this question. AB is a helix of 

 covered copper wire one-fifteenth of an inch in thickness : the 

 length of the helix is 5 inches, external diameter 5 inches, and 

 internal diameter 1"5 inch. Within this helix a bar of bismuth 

 6i inches long and 0'4 of an inch thick was svispended. The 

 suspension was effected by means of a light beam, from two 

 points of which, sufficiently distant from each other, depended 

 two silver wires each ending in a loop : into these loops, //', the 

 bar of bismuth was introduced, and the whole was suspended by 

 a number of fibres of unspun silk from a suitable point of sup- 

 port. Fig. 39 a is a side view of the arrangement used for the 

 suspension of the bar. Before introducing the latter within the 

 helix, it was first suspended in a receivei*, which protected it from 

 air currents, and in which it remained until the torsion of the 

 fibre had exhausted itself: the bar was then removed, and the 

 beam, without permitting the fibre to twist again, was placed 

 over the helix so as to receive the bar introduced through the 

 latter. From the ends of this helix two wires passed to a cur- 

 rent reverser R, from which they proceeded further to the poles 

 of a voltaic battery. CD and EF are two electi'o-magnetic spirals, 

 each 12 inches long, 5i inches external and 2 inches internal 

 diameter. The wire composing them is one-tenth of an inch 

 thick, and so coiled that the current could be sent through four 

 wires simultaneously. Within these spirals were introduced two 

 cores of soft iron 2 inches thick and 14 inches long : the ends 

 of the cores appear at V and P'. The spirals were so connected 

 together that the same current excited both, thus developing the 

 same magnetic strength in the poles P P'. From the ends of the 

 spirals proceeded wires to the current rcverser R', and thence to 

 a second battery of considerably less power than the former. 

 By means of the rcverser 11' the polarity of the cores could be 

 changed ; P' could be converted from a south pole to a north 

 pole, at the same time that P was converted from a north pole 

 to a south pole. Lastly, by a change of the connexions between 

 the two spirals, the cores could be so excited as to make the 

 poles of the same quality, both north or both south. 



Tlie diameter of the cylindrical space, within which the bis- 

 muth bar was suspended, was sucb as to permit of a free play of 

 the ends of the bar through the space of an inch and a half. 

 Having seen that the bar swung without impediment, and that 

 its axis coincided as nearly as ])ossible with tlu^ axis of the helix, 

 a current from the battery was sent through the latter. The 

 magnetism of the cores P and 1" was then excited, and the action 



