CHAP, v.] ELECTRICITY AND MAGNETISM. 295 



has shown 1 that in the case of two parallel concurrent 

 circuits the " lines of force " due to the two systems 

 run into one another, embracing both circuits, while in 

 the case of two parallel and non-concurrent circuits the 

 " lines of force " due to the two currents indicate mutual 

 repulsion. The theory of Maxwell, that a voltaic circuit 

 acts like a magnetic shell (a direct deduction from Fara- 

 day's work), is in practice a more fruitful conception than 

 that of Ampere. On Maxwell's theory two circuits will 

 tend, like two magnetic shells, to move so as to include 

 as many of one another's " lines of force " as possible 

 (Art. 193 and 320). This will be the case when they 

 coincide as nearly as possible ; i.e., when the two wires 

 are parallel in every part, and when the currents run 

 round in the same direction. In fact, all the electro- 

 dynamic laws of parallel and oblique circuits can be 

 deduced from Maxwell's theory in the simplest manner. 

 An interesting experiment, showing an apparent 

 mutual self-repulsion between contiguous portions of the 

 circuit, was devised by Ampere. A trough divided by 

 a partition into two parts, and made of non-conducting 

 materials, is filled with mercury. Upon it floats a 



Fig. 123. 



metallic bridge formed of a bent wire, of the form shown 

 in Fig. 123, or consisting of a glass tube filled siphon- 

 wise with mercury. When a current is sent through 

 the floating conductor from X over MN, and out at 



1 Philosophical Magazine, November 1878, p. 348. 



