4 68 



PARTICULAR CASES. 



two consecutive teeth, and suppose the surface of the mercury 

 placed so that one of the teeth touches the liquid at the moment the 

 preceding one quits it. The flow of force through the triangle 

 formed by the radius, which corresponds to these two teeth, is 



F , or sensibly F ; that is to say, the product of the force 



2 aW 



by the surface of the sector, and the corresponding work is IF . 



For an entire turn the work is IFS that is to say, proportional to 

 the whole surface S of the wheel. 



484. 2nd. Ampere's Experiment. This experiment, in which 

 the rotation of a magnet is produced by a current, is the converse 

 of that, the theory of which has been given above (456). The 

 apparatus is arranged so that only one of the poles of the magnet 

 can traverse the current ; a continuous rotation is obtained in this 

 way. The magnet (Fig. 112), loaded by a counterpoise of platinum, 

 floats on the mercury, and can rotate about itself on its own axis ; 

 the current is brought to the surface of the liquid, traverses the 

 projecting part of the magnet, and emerges by a fixed conductor 



Fig. 112. 



which dips in a drop of mercury in the top N. If we suppose that 

 the current goes rigorously along the axis of the magnet, the work at 

 each turn for a magnetic mass outside the axis is 47r;;/I, and gives 

 rise to a couple the moment of which is 2ml. The phenomenon is 

 really more complicated, because the current traverses the whole 

 section of the magnet. 



Faraday repeated the experiment by placing the magnet outside 

 the circuit. The magnet is brought to the centre of the vessel by a 

 metal rod, and the magnet floats in an eccentric position. 



In both cases, if the current ascends by the axis, and the top of 



