ELECTRO-MAGNETISM* 



the forces are in equilibrium. The 

 nature of this equilibrium will appear 

 from fig. 94, in which the circle is repre- 

 sented as seen from above ; and the 

 arrows point out the horizontal direction 

 of the forces, which when the circle is in 

 the line W E pass through the vertical 

 axis and are exactly balanced. The same 

 letters denote the corresponding points 

 in the two figures. 



Fig. 94. 

 N 



clecfro-magnefic force of the earth, 

 tends to arrange itself in the plane of 

 the magnetic equator. If at liberty to 

 do so, it will assume this position ; but 

 if its motion be restricted to any other 

 directions, it will place itself as nearly 

 as possible in that plane. If, for in- 

 stance, it be constrained by a vertical 

 axis to turn horizontally only, it will 

 place itself in a plane perpendicular to 

 that of the magnetic meridian ; that is, 

 directed east and west, or fronting to 

 the north and south. In its movements 

 to attain this position, it is urged by the 

 tendencies of the currents, in as far as 

 their motion is vertical, whether ascend- 

 ing or descending ; for the tendencies of 

 the horizontal portions exactly balance 

 one another, and produce no rotatory 

 effect. Thus if in the circle AD, Jig. 93, 

 which turns on the vertical axis, X Y, 

 the current descend in the branch D, 

 and ascend in the branch A, the former 



will be impelled in the direction D E, 

 and the latter in the direction A W, by 

 forces which cease only when they have 

 attained the positions E and W. The 

 horizontal portions at X and Y neu- 

 tralize each other, with regard to their 

 rotatory tendency. Hence the circle 

 will revolve until it takes the position 

 shown by the circle at E and W, where 



('Hf>.) When, on the other hand, the 

 axis of revolution is horizontal, and the 

 motion vertical, the position is deter- 

 mined by the forces that act. on the 

 horizontal parts of the circle, which now 

 conspire in determining a rotation to- 

 wards the plane of the magnetic equa- 

 tor, if the plane of motion coincide with 

 the magnetic meridian; or if not, as 

 near to it as the restrictions to the mo- 

 tion will allow. 



(147.) On the whole, then, it appears 

 that a heliacal coil, such as that de- 

 scribed in 105, balanced on its centre, 

 will assume all the positions, and ex- 

 hibit all the directive properties of the 

 magnetic needle. 



CHAPTER X. 

 Electro- Magnetic Induction. 



(148.) THUS far we have considered 

 the electro-magnetic phenomena that 

 result from the reciprocal action of gal- 

 vanic currents on magnetized bodies. 

 We have next to examine that class of 

 effects which arise from the action of 

 the former on iron, or other ferruginous 

 bodies that have not previously been 

 rendered magnetic. Experiment has 

 proved that a conducting wire, during 

 the passage of an electric current 

 through it, tends to induce magnetism 

 in such bodies as are in the vicinity, and 

 in which that state is capable of being 



