ELECTRO-MAGNETISM. 



relation between the dip and the moving 

 wire *. 



(14-2.) It is evident that by restraining 

 the motion of one of the ends of the 

 horizontal wire in the experiment de- 

 scribed in 137, and represented in 

 fig. 88, so as to render that end a fixed 

 axis, and providing a circular mercurial 

 trough for the other end to move in, 

 the same force which produced a paral- 

 lel progressive motion in the former 

 case, will now produce a rotatory mo- 

 tion ; because the force producing a 

 horizontal motion is the same in all 

 positions of the wire. This equality is 

 proved by making the experiment with 

 two connected horizontal wires instead 

 of one ; placing them the one imme- 

 diately above the other, each being 

 furnished with its separate mercurial 

 trough, into which the moveable end 

 may dip. This arrangement is repre- 

 sented in fig. 91, where the current 

 entering by the cup P, and travers- 

 ing the mercury in the upper trough, 

 ascends through the wire A, passes 

 on through the upper horizontal wire 

 to the central wire C, placed in the 



Fig, 91. 



axis of suspension, (Ihe wire being hung 

 by the slender thread S,) alon<r which 

 it descends, and passes outwardly 

 through the lower horizontal wire, and 

 thence through the mercury in the 

 lower trough, to the cup N, whence it 

 escapes to the voltaic battery. When 

 this has been effected, it is found that 

 the suspended wires exhibit no tendency 

 to rotate, in whatever azimuth they may 

 be placed. Hence it may be inferred 

 that the tendency to revolution which 

 the earth communicates to the current 



* Quarterly Journal of Science, XII, 413 



moving from the circumference to the 

 centre in the upper horizontal wire, is 

 exactly counterbalanced by an opposite 

 rotatory force in the lower wire, in which 

 the current passes from the centre to 

 the circumference; and as this equality 

 is preserved in every azimuth, it follows 

 that the rotatory force is constant in 

 every position of the wire. 



(143.) A vertical current in a con- 

 ductor moveable round a vertical axis 

 is also impressed by the influence of the 

 earth with a horizontal force, which 

 carries it towards the magnetic east, 

 when the current is descending, and 

 towards the west when it is ascending. 

 This will be made apparent by suspend- 

 ing a wire bent as shown in fig. 92, A, 

 and terminating above and below in the 

 points P and N, for the purpose of being 

 placed in their respective cups, and 



Fig. 92, A. 



balanced by a counterpoise, L. When 

 the current is passed through this wire, 

 the direction of its course along^he two 

 horizontal branches, H and h, being op- 

 posite, will counterbalance each other ; 

 but that in the vertical portion, V, will 

 take a position either to the east or west 

 of the axis, according as the current 

 descends or ascends through it. 



(144.) If the wire, instead of having 

 the shape just shown, has the figure of 

 a complete square or parallelogram, as 

 shown in Jig. 92, B, the second vertical 

 branch will conspire with the first in 

 making the wire assume the same po- 

 sition, namely, that in which the current 

 descends, (as V,) to the east, and that 

 in which it ascends, (v,) to the west. 



(145.) From an attentive considera- 

 tion of the facts that have now been 

 stated, we are enabled to understand why 

 a vertical circular current, urged by the 

 E2 



