ELECTRO-MAGNETISM. 



37 



be determined to the ri^lit, and the 

 south pole to the left ; and this tendency 

 will be given in the right-handed helix 

 if the current be transmitted through it 

 from left to right ; but in the left-handed 

 helix from right to left. It requires but 

 a slight effort of attention to these par- 

 ticulars to perceive the influence they 

 have on the phenomena ; yet unless this 

 effort be made mistakes may easily be 

 committed. 



(103.) When the needle lies exactly 

 in the middle of the axis of the helix, 

 the opposite forces which impel the two 

 poles in contrary directions, derived 

 from each coil of the wire, exactly 

 balance one another, and the needle re- 

 mains in equilibrium. When disturbed 

 from this position, by being pushed 

 nearer to one end, the forces derived 

 from the turns of the wire collectively 

 act with more power upon that pole 

 which is nearest to the middle point of 

 the axis, both because they are nearer, 

 and because they act less obliquely. 

 These forces will, therefore, prevail over 

 those that urge the more distant pole in 

 the contrary direction; and the magnet 

 will be brought back to its former po- 

 sition in the" middle of the axis. This 

 is illustrated mfig. 70, which represents 

 a section of the helix; S N being the 



Fig. 70. 

 o. o o o o o. o o o_.o o o o o 0,0 



secondly, because they act with less ob- 

 liquity : they will therefore impel the 

 whole magnet towards the middle of the 

 axis. 



(104.) So powerful is the action of a 

 helix of this description, that if a small 

 magnetized needle, or bar, be placed 

 within it, so as to rest upon the lower 

 portions of the wire, the moment the 

 connexion is made with the Voltaic 

 battery, so that the electric current 

 circulates through the wires, the needle 

 is seen to start up, and place itself in 

 the axis, remaining suspended in the 

 air in opposition to the force of gravity. 

 This will even take place in a vertical 

 position of the helix, presenting the 

 singular spectacle of a heavy body raised 

 by an invisible power, and maintained, 

 like the fabled statue of Theamides, in 

 a situation totally free from any ma- 

 terial connexion and support. 



(105.) The magnetic actions of a 

 helix at its two extremities, and at some 

 distance beyond them, agree with those 

 of the sides of a single circle, or spiral 

 coil already explained ; one end having 

 properties similar to the north, and the 

 other to the south pole of a magnet. 

 But the imitation may be rendered still 

 more complete if the two portions of the 

 wire which has formed the helix, and 

 are situated at its two extremities, be 

 bent back as shown mjig. 71, at N, S, 

 so as to return in a straight course 

 along the axis till they arrive at the 

 middle point, where they are again bent 

 at right angles, in order to pass out 

 between the coils, rising parallel to one 



position of the magnet, a little to 

 side of the middle point of the axis, 

 will be evident that, in as far as the 

 S is acted upon by forces derived 

 the turns of the wire situated between 

 A a and C c, its tendency to move\ut 

 wards is exactly balanced by the 

 arising from the action of the wires be 

 tween B b and D d upon the pole N, 

 urging it in the contrary direction; 

 because these wires have exactly the 

 same relative situations to these re- 

 spective poles. But the pole N is be- 

 sides acted upon by all the wires that 

 are situated between B b, and the end 

 A a, and the pole S by all those situ- 

 ated between C c and D d. These two 

 actions are in opposite directions; but 

 the former is more powerful than the 

 latter; first, because the wires between 

 Aa and B b are nearer to N, than those 

 between C c and 1) d are to S ; and 



another, and terminating in points for 

 the purpose of suspension in cups, as 

 already described in the case of the 

 spiral wire. Sometimes one of these 

 wires, instead of being bent upwards, is 

 made to descend vertically, arid ter- 

 minate in a sharp point below, where it 

 is inserted into a cup. 



(106.) What constitutes the peculiar 

 excellence of this arrangement which 



