198 Historical Sketch of Electro-magnetism. [Sept. 



sion of its opposite ends ; and it is the mode of suspension 

 combined with the earth's magnetic power that prevents it 

 from traversing; in a vertical circle. But if those interfering: 

 circumstances be removed, i. e. if the suspension be such as to 

 allow of free motion to the needle in every direction, and the 

 earth's magnetism be rendered null, or counteracted either bv 

 the position of the needle, or by the vicinity of another magnet, 

 then a much simpler idea of the relative movements of the wire 

 and needle may be obtained. 



It is not, perhaps, easy to obtain this perfect state of the 

 apparatus, but it is not difficult so to arrange it as to examine 

 the movements first in one direction, and then in another. It 

 will then be found that if the connecting wires of a sufficiently 

 powerful apparatus be placed near a magnetic needle so as to 

 pass near its centre, that the needle will arrange itself directly 

 across the wire, whatever the previous position of the two; that 

 if the wire be carried round the centre of the needle, or the cen- 

 tre of the needle round the wire, the same relative position of 

 the two will continue, and that the direction of the needle across 

 the wire is not indifferent, but has its poles always in a constant 

 position to the poles of the battery. If the positive pole of a 

 battery be on our right hand, and the negative pole on the left, 

 and a wire be stretched between, connecting them, then a needle 

 above the wire will point the north pole from, and the south 

 towards ; or if below, the south pole from, and the north towards 

 us (Plate IX *) ; figs. 1, 2. If the connecting wire and the 

 needle be represented by two small rods named accordingly, and 

 fastened permanently together, then they will represent the wire 

 and the needle in all positions ; for, however one be placed, the 

 other will correspond with it : or if on the under side of a small 

 square piece of glass a line be drawn from top to bottom, the 

 upper end being called negative, and the lower positive ; and on 

 the upper surface a line be drawn from left to right, the left ter- 

 mination being named south, the right north ; then the lower 

 line will always represent the connecting wire, and the upper 

 the needle; fig. 3. 



The needle and wire being in this position, if the wire be 

 moved along the needle towards either extremity, strong attrac- 

 tion will take place between it and the pole, notwithstanding the 

 same part of the wire be employed; and the poles in the two 

 positions are contrary to each other. In this case it appears 

 that the same point in the wire has the power of attracting both 

 the north and south pole of the needle. If, while the wire is 

 thus situated near the end of the needle, the latter be turned 

 round so that the pole before there be replaced by the opposite 

 pole, strong repulsion will take place, and that to whichever pole 

 the wire has in the first instance been carried ; so that the same 



• This Plate will be given in the next Number. 



