FARADAY'S EXPERIMENTS. 



435 



poles and the other without, and let us suppose that a current goes 

 from the point A, to the point B, by the arc ACB. The flow of 

 magnetic force from the pole P, which traverses the portion ACB of 



P U 



Fig. 101. 



the circuit, seems null, for the pole is in the plane of the circuit ; 

 and, moreover, the arc ACB seems in all azimuths to have an 

 identical position in reference to the pole. Yet the arc ACB acquires 

 a continuous rotatory motion, which, if the pole P is a North pole, 

 makes it turn in the direction of the hands of a watch for an 

 observer placed above the point A. 



In order to analyze this phenomenon let us replace the current 

 by the equivalent shell ; we may imagine that the movable part of 

 this shell is made up of an infinitely elastic plate, which forms a 

 concave surface behind the pole, and presents its negative face to the 

 pole. As this surface tends to comprise a great part of the flow, it 

 will move in the direction indicated, and the motion will be con- 

 tinuous, as the elastic sheet can fold upon itself indefinitely. 



The change in the flow of the force for a rotation of the plane 

 of the current, will be equal to 2mO ; the work of the electromagnetic 

 forces relatively to this pole will be 2mOl for the displacement 0, and 

 for a complete turn, 4ir;//I. 



The moment of the couple of rotation in reference to the axis is 

 then expressed by 



477 ml 



27T 



2ml ; 



it is to be observed that this moment is independent of the magni- 

 tude and shape of the arc. 



F F 2 



