CHAP, v.] ELECTRICITY AND MAGNETISM. 293 



consisting of a double supporting stand, upon which 

 conductors formed of wire, shaped in different ways, can 

 be hung in such a way as to be capable of rotation. 

 In the figure a simple loop is shown as hung upon the 

 supports. The ends of the wires of the movable 

 portion dip into two mercury cups so as to ensure good 

 contact. The solenoid, Fig. 1 16, is intended to be hung 

 upon the same stand. 



By the aid of this piece of apparatus Ampere further 

 demonstrated the following points : 



(a) A circuit doubled back upon itself, so that the 

 current flows back along a path close to itself, 

 exerts no force upon external points. 



(b) A circuit bent into zig-zags or sinuosities, pro- 

 duces the same magnetic effects on a neigh- 

 bouring piece of circuit as if it were straight. 



(c) There is in no case any force tending to move a 

 conductor in the direction of its own length. 



(d) The force between two conductors of any form is 

 the same, whatever the linear size of the system, 

 provided the distances be increased in the same 

 proportion, and that the currents remain the 

 same in strength. 



The particular case, given in Fig. 122, will show the 

 value of these experiments. Let AB and CD represent 

 two wires carrying currents, lying neither parallel nor in 

 the same plane. It follows from (), that if we replace 

 the portion PQ by the crooked wire PRSQ, the force 

 will remain the same. The portion PR is drawn verti- 

 cally downwards, and, as it can, by (c), experience no 

 force in the direction of its length, this portion will 

 neither be attracted nor repelled by CD. In the portion 

 RS the current runs at right angles to CD, and this 

 portion is neither attracted nor repelled by CD. In the 

 portion SQ the current runs parallel to CD, and in the 

 same direction, and will therefore be attracted down- 



