ELECTRO-MAGNETISM. 



extended current upon the one that is 

 terminated, an equal but contrary re- 

 action is necessarily exerted by the ter- 

 minated one upon the extended current. 

 Thus, while the current A B, fig. 114, 



Fig. 114. 

 A 



UG. 



]J 



tends to move in the direction of the 

 upper arrow, its reciprocal action on the 

 current C D, urges the wire that con- 

 ducts it in the contrary direction, from 

 D to C, as denoted by the lower arrow ; 

 and the same principle applies to all the 

 other cases. 



$ 4. Action of Diverging and Converg- 

 ing Currents. 



(204.) Since the rotatory force is the 

 same in all positions of the wire, it is 

 evident that if wires, or other conducting 

 bodies, be so disposed as to cause the 

 currents to radiate from a centre in all 

 directions, they will tend to revolve in 

 the same manner as any one of them 

 singly would have done, by the influence 

 of a rectilineal current in the vicinity, 

 and in the same plane ; provided this 

 latter current be wholly without the 

 circumference of the circle of revolution. 

 The same thing will happen when the 

 currents converge from the circumference 

 to Hie centre, only the direction of the 

 motion will be reversed. These two 

 conditions of the experiment are repre- 

 sented in figs. 115 and 116, where the 

 arrow-heads in the paths of the cur- 

 rents denote their direction ; and the 

 exterior dotted arrows the direction of 

 the revolution of the conductors. 



Fig. 115. 



(205.) Examples of this kind of di- 

 vergence or convergence of currents 

 frequently occur in electro- dynamical 

 experiments. They are met with when- 

 ever a fluid conductor, such as mercury, 

 is the medium of communication be- 

 tween the point of a conducting wire 

 dipped into the fluid and a circular rim 

 of metal ; in which case there is always 

 more or less of diffusion of currents 

 while they are passing through the fluid ; 

 and generally there is a tolerably regu- 

 lar radiation or concentration of the 

 currents, of which some idea may be 

 formed by fig. 117; where the current 

 passing from P to N, through mercury 

 contained in the cylindrical vessel V, 

 will radiate from the point of the wire 

 towards every part of the circumference 

 of that vessel. If the current pass from 

 N to P, it will converge towards the 

 wire. In either case the action of a 

 strong current, passing along the straight 

 horizontal conductor C D, will give rise 

 to a revolving motion in the mercury, the 

 direction of which, corresponding with 

 the directions of the two currents, is 

 indicated by the arrows at w, D, and m. 

 (206.) It is not easy to exemplify by 

 direct experiment the theoretical de- 

 ductions applicable to the case of the 

 action of a straight current of indefinite 



