30 



ELECTRO-MAGNETISM. 



r. GO. 



into the form of a spiral, see fig. 66. 

 The upper end of the wire should be 

 bent downwards and terminate in a 

 point, for the purpose of being inserted 

 in a cup containing mercury, which 

 communicates by a wire with one of the 

 poles of the Voltaic battery. The coils 

 of the wire may be either secured from 

 contact by being wrapped round with 

 silk thread, or may be attached to one 

 surface of a card, while the wire which 

 proceeds from the centre of the coil 

 passes through the card, and descends 

 in a straight line on the opposite 

 side, so as to rest by its pointed extre- 

 mity on the inside of another cup, also 

 containing mercury, in order to form 

 a communication with the other pole 

 of the battery. A coil of this descrip- 

 tion, all the successive coils of which 

 conspire together in producing the op- 

 posite polarities on its two sides, imitates 

 still more decidedly the effects of a 

 magnet, whose poles might be supposed 

 to be situated in the centre of each disc. 



(101.) A still closer imitation of a 

 magnet is obtained by making the turns 

 of the wire not in the same plane, as in 

 the spiral just described, but on a cylin- 

 drical surface, like the turns of a cork- 

 screw ; a figure which mathematicians 

 have termed a helix : an arrangement 

 which possesses many remarkable pro- 

 perties, both as regards its interior and 

 its exterior action. 



In fig. 67, the several turns of the 

 helix are represented as separated to a 

 distance from each other, in order that 

 the direction of the turns, and the po- 

 sition of a magnet placed in the axis 

 may be distinctly seen. The electro- 



magnetic influence exerted by each turn 

 is, as we have seen, to urge the north 



Fig. 67. 



pole of a magnet placed in its axis, to 

 move in one direction along that axis, 

 and the south pole in the contrary di- 

 rection. The force thus exerted is, of 

 course, multiplied in degree and in- 

 creased in extent, by each repetition of 

 the turns of the wire; and a magnetic 

 needle in every part of the interior of the 

 helix will have a powerful tendency to 

 place itself in the axis, and to turn its 

 poles in a manner conformable to the 

 nature of the force that is in operation. 

 (102.) Now this force depends on two 

 circumstances : first, the direction of the 

 current with reference to the axis of the 

 helix ; and, secondly, the direction of 

 the circumvolutions which compose it. 

 It is well known that screws are of two 

 kinds, distinguished as right-handed 

 or left-handed screws. In the former, 

 as shown in fig. 68, the turns proceed 



Fig. 68. 



Fig. 69. 



downwards, (if the screw be placed with 

 its axis vertical) from right to left, on 

 that side which is next to the spectator. 

 In the left-handed screw (see fig. 69), 

 the turns proceed in the contrary direc- 

 tion. Now the magnetic polarity of the 

 electric helix, which is exerted in the 

 space it encircles, depends on the direc- 

 tion in which the current is moving with 

 reference to a plane at right angles to 

 the axis; for if the current be descend- 

 ing on the side next to the spectator, 

 (in the horizontal helix, fig. 67,) the 

 north pole of a magnet in the axis will 



