ELECTRO-MAGNETISM. 



contrariety of effect will, on the other 

 hand, be removed, if the straight cur- 



Fig. 133. 



rent be wholly exterior to the circle, as 



A B, fig. 134, though still moveable 



round the same axis as before. The 



revolution will now be performed in a 



Fig. 134. 



A 



direction contrary to that of the interior 

 current in the former case. The reaction 

 of an exterior current on the circular 

 conductor will likewise be in the oppo- 

 site direction to what it was before. 



(225.) It is obvious that every thing 

 that has been stated with regard to 

 straight currents, the direction of which 

 is towards or from the centre of the 

 circular current, applies also to a num- 

 ber of radiating or converging currents. 



Hence if the cylinder,}?^. 129, com- 

 municating by its point with one of the 

 poles of a voltaic battery, have its lower 

 rim immersed in a flat dish containing 

 mercury, communicating by a wire from 

 its centre with the opposite pole of the 

 battery, radiating or converging cur- 

 rents will be established in the mercury, 

 which, acting on the vertical currents 

 existing in the sides of the cylinder, will 

 cause it to revolve. If the currents 

 tending to or from the cylinder be ex- 



terior to it, which may be obtained by 

 surrounding the cylinder with a metallic 

 ring of larger diameter, and making this 

 ring the medium of communication with 

 the battery, the revolution of the cylin- 

 der will be made in the opposite direc- 

 tion to what it was before. 



7. Reciprocal Action of Circular 

 Currents. 



(226.) The mutual actions exerted 

 between two circular currents, may 

 readily be collected from the application 

 of the general law of attraction among 

 those parts in which the directions of 

 the currents are similar, and of repul- 

 sion where they are dissimilar. If one 

 of the circles be fixed and the motion of 

 the other be limited to revolution round 

 an axis, the effects of their mutual 

 action will depend on the position of 

 the centre of the moveable circle with 

 regard to the plane of the fixed circle, 

 and also upon the position and inclina- 

 tion of the axis with relation to the line 

 joining both centres. 



(227.) If the centre of the moveable 

 circle be in the same plane with the 

 fixed circle, or not far removed from it, 

 whatever be the inclination of the axis, 

 a directive force will arise, tending to 

 bring the whole of the circumference 

 into that plane, and to make it assume 

 such a position as that the currents in 

 the adjacent portions of the circle shall 

 be in the same directions. Thus C D, 

 (figs. 135 and 136,) being the fixed, 



Fig. 135. 



and A B the moveable circle, on the 

 axis X Y, the directive force will bring 

 the latter into the position E e, that is, 

 in the same plane with C D ; and this will 

 happen equally, whether the axis be at 

 right angles to the line joining their 



