ELECTRO-MAGNETISM. 



perpendicular to this line, it must be 

 situated in the plane of the magnetic 

 equator. Such, then, is the direction 

 in which it receives the full influence of 

 the earth's magnetism ; and this influ- 

 ence is exerted in urging it to move in 

 a direction parallel to itself, and at the 

 same time perpendicular to the line of 

 the dip ; that is, it tends to continue in 

 the plane of the magnetic equator. The 

 direction of its motion to the one side 

 or the other must depend altogether 

 upon the course of the electric current 

 which is passing through it. If the 

 wire, for example, be placed horizontally, 

 and have the direction of the magnetic 

 east and west, and the current of posi- 

 tive electricity be flowing through it 

 from west to east, the tendency to mo- 

 tion in the wire, in consequence of the 

 influence of the earth, which acts like a 

 south pole, is towards the north, that 

 is ascending in the plane of the mag- 

 netic equator, which plane, it may be 

 recollected, dips downwards towards the 

 south, with an inclination to the horizon 

 of about twenty degrees, equal to the 

 complement of the dip. 



This will be more clearly under- 

 stood by reference to fig. 86, in which 

 N, E, S, W represents a horizontal 

 plane. D d, which has an inclination 

 to this plane of 70, is the line of dip, 

 to which the plane M M, representing 

 the plane of the magnetic equator, is 

 perpendicular. W E is a straight por- 

 tion of conducting wire, along which an 



Fig. 86. 



electric current is flowing in the direc- 

 tion from W to E. Under these cir- 

 cumstances, the effect of the electro- 

 magnetic force exerted by the earth is 

 to give the wire a tendency to move 

 parallel to itself, in the plane M JR, and 

 towards M, as denoted by the arrow ; 

 so that were it at liberty to obey this 

 impulse, it would next be found to 



occupy the position marked by the dotted 

 line we. If the electric current had 

 been made to pass from E to W, the 

 direction of the motion would have 

 been altered, and the wire would have 

 moved downwards in the same plane, 

 still, however, preserving its paral- 

 lelism. 



(136.) If the wire extend in the mag- 

 netic plane from north to south, as, 

 for instance, along the line M yE in 

 fig. 87, N S, as before, being the hori- 

 zontal plane: and if the electric cur- 

 rent move in the direction M M, that is, 



Fig. 87. 



from north to south, the wire will tend 

 to move towards the east, as shewn by 

 the arrow, still keeping in the same 



flane, and remaining parallel to itself, 

 f, on the contrary, the current move 

 from south to north, the wire will be 

 impelled to move from east to west. 



It need hardly be observed that all 

 these statements relate to what happens 

 in the northern magnetic hemisphere of 

 the earth, and when the dip is about 70 

 degrees, as is the case in England. In 

 the southern hemisphere, where the 

 northern polarity of the earth is in ac- 

 tivity, the effects are of course reversed. 

 At the magnetic equator, where the dip 

 is nothing, the plane M JE is perpendi- 

 cular to the horizon, and the tendency 

 to motion of a horizontal wire must be 

 directly upwards, or directly downwards ; 

 and the effect of the terrestrial mag- 

 netism must be merely that of opposing 

 or conspiring with gravitation, that is of 

 producing either an increase or a diminu- 

 tion in the apparent weight of the wire. 

 In these high latitudes the inclination of 

 the magnetic equator to the horizon is 

 too small to produce any very sensible 

 effect of this kind. It has, however, 

 been rendered perceptible in very nice 

 experiments. 



(137.) In consequence of the plane of 

 the magnetic equator being not very far 

 removed from a horizontal plane, wires 

 placed horizontally, and being free to 

 move in a horizontal plane, may be 

 made to exhibit the actions of terrestrial 

 magnetism without much difficulty. Mr, 



