ELECTRO-MAGNETISM. 



part. This is shown in fig. 155, which 

 represents one of these elementary mag- 

 netic filaments, the eastern side being 

 presented to the spectator. 



Fig. 155. 



(268.) These currents which exist in 

 each particle of a magnet, may there- 

 fore be considered as constituting closed 

 circuits (see $ 232), the effects of which 

 on all bodies exterior to the circuit will 

 depend on the difference between the ac- 

 tions of the nearest and the most remote 

 parts of the circle described by the cur- 

 rent. The united effects of a great num- 

 ber of these circular currents will almost 

 entirely depend on these parts of the 

 current which occupy the exterior sur- 

 face of the mass. 



(269.) Thus, supposing the magnet 

 to be cylindrical in its shape, and its 

 section shown in fig. 156, to consist of 

 the sections of each of its component fila- 

 ments a, b, c, d, &c., and round each of 



Fig. 156. 



which electric currents are circulating in 

 the directions indicated by the arrows, it 

 is evident that the currents of all the 

 interior parts will nearly, if not exactly, 

 compensate one another, and that their 

 action will be neutralized. But the cur- 

 rents that pass near the circumference 

 are differently circumstanced, inasmuch 

 as they are not compensated by any 

 others ; and their action is, therefore, 

 fully exerted on the bodies that are near 

 them, and is equivalent to that of a 

 single circular current flowing uniformly 

 round the circumference, p, p, p, of a 

 circumscribing circle, in the same direc- 

 tion. Hence, in estimating the effects 

 of the whole assemblage, we may con- 

 fine our attention to that of a superficial 

 current. 



(270.) It is obvious that in order to in- 



stitute an exact comparison between the 

 action of a magnet, and that of an artifi- 

 cial assemblage of electric currents simi- 

 lar to that which is supposed by the 

 theory to exist in the magnet, our imi- 

 tation must be made by collecting toge- 

 ther a great number of similar helices, 

 in parallel directions, and uniting them 

 in one mass Such an arrangement is 

 called by Ampere an Electro-dynamic 

 Solenoid *. 



(271.) The tendencies which a magnet 

 and conducting wire have to place them- 

 selves in positions at right angles to 

 one another, was deduced from the elec- 

 tro-magnetic theory as a consequence 

 of the supposed transverse situation of 

 magnetic fluids resulting from the elec- 

 tric conflict that is, accompanying the 

 movements of the electric fluids. In 

 Ampere's theory the trans-verse direc- 

 tion of the action is ascribed to the 

 transverse movement* of the electric 

 currents in the magnet itself, which act 

 upon the current in the conductor, and 

 are also acted upon by that current, and 

 tend constantly to establish a parallelism 

 between them. Thus, since the currents 

 in the magnet N S, Jig. 157, move in 

 planes perpendicular to the axis of the 

 magnet, their action, being in those 

 planes, is transverse to the axis, and 

 tends to bring a straight conduct ing- 



Fig. 157. 



wire, PQ, into the transverse position 

 represented in the figure, in which the 

 direction of the current of the conduc- 

 tor is parallel to that of the current in 

 the nearest part of the magnet. On the 

 other hand, if the wire be fixed, and the 

 magnet moveable, the forces will tend 

 to bring the plane of that current, which 

 occupies the middle of the magnet, into 

 such a position as may include the 

 straight conductor ; and as the axis of 

 the magnet is perpendicular" to that 

 plane, so also must it be at right angles 



* Tbcorie des Phenouilnes Electro-dynaimciuesj 

 p. 95, 



