296 ELEMENTARY LESSONS ON [CHAP. v. 



Y, the floating bridge is observed to move so as to 

 increase the length of the circuit. But Maxwell has 

 shown that the true explanation depends upon the self- 

 induction (Art. 404) of the two parallel portions of the 

 floating conductor, and that the force would be diminished 

 indefinitely if the two parallel parts could be made to 

 lie quite close to one another. 



335. Electromagnetic Rotations. Continuous 

 rotation can be produced between a magnet and a 

 circuit, or between two parts of one circuit, provided 

 that one part of the circuit can move while another part 

 remains fixed, or that the current in one part can be 

 reversed. The latter device is adopted in the construc- 

 tion of the electromagnetic engines described in Art. 375 ; 

 the former alternative is applied in a good many interest- 

 ing pieces of apparatus for showing rotations, a sliding- 

 contact being made between one part of the circuit and 

 another. Several different forms of rotation-apparatus 

 were devised by Faraday and by Ampere. One of the 

 simplest of these is shown in Fig. 1 24, in which a 



Fig. 124. 



current rising through a and passing through the lightly 

 pivoted wire b b' in either direction, passes down into 

 a circular trough containing mercury. The trough is 

 made of copper, and is connected with a wire which is 

 also wound in a coil round the outside of the trough, 



