ELECTROMAGNETIC ROTATION 7 ". 



467 



Ampere thought this a proof that the two elements of current 

 directed along the same right line, and in the same direction, repel 

 each other, as the elementary formula indicates; but it is easy to see 

 that the interpretation of the phenomena does not entail this conse- 

 quence. 



In this experiment the current traverses a circuit, one of whose 

 portions is movable, and the surface of which tends to become a 

 maximum (455). This result may moreover be arrived at directly 

 by replacing the current by a flexible shell bent upon itself, as 

 shown in Fig. in. The three shells superposed in the space 

 ABB'A' do not give rise to any force among them parallel to the 

 plane of the current ; but their external action is equivalent to that 

 of a simple shell. The portion aDC& tends to recede, and the shell 

 extends so as to occupy the greatest surface. 



Fig. in. 



483. ELECTROMAGNETIC ROTATION. ist. Barlow's Wheel 

 A toothed metal wheel, movable about a horizontal axis, is arranged 

 so that one or more teeth plunge with their lower ends in a cup 

 containing mercury. If the system is traversed by a current 

 which enters by the axis and leaves by the mercury, the only 

 action of the current on itself will tend to move the bottom 

 teeth in a direction which displaces them from the rest of the 

 circuit so as to increase the total surface; but this action is 

 generally too weak to overcome friction. A stronger effect is ob- 

 tained by putting the trough between the limbs of a horseshpe 

 magnet arranged horizontally. The lines of magnetic force then 

 traverse the plane of the wheel ; if they are directed from back 

 to front that is, with the north pole in front the rotation will 

 be in the opposite direction of the hands of a watch. 



In order to get a phenomenon easy to calculate, replace the 

 magnet by a uniform magnetic field of intensity F, parallel to the 

 axis of the rotation. Let a be the radius of the wheel, 6 the angle of 



H H 2 



