ACTION OF A UNIFORM FIELD. 471 



the movable bar, the variation in the flow of magnetic force in the 

 circuit is bUx. In the case of the terrestrial field, and if the rails 

 are horizontal, the component Z is directed downwards in our hemi- 

 sphere, and with the direction of the current shown by the arrows, 

 the movable bridge CC' will recede from AB under the influence of 

 electromagnetic forces ; it will approach, if the current is in the 

 opposite direction. 



In Ampere's experiment (482) the action of the current on itself 

 tends to increase the surface, and to repel the movable bridge. 

 This action and that of the Earth will add themselves or oppose 

 according to the direction of the current ; the motion of the wire is 

 more or less easy according to the case. 



A" 



y i 



Fig. 114. 



488. Suppose now that the movable conductor forms a closed 

 circuit ; let S be the surface of this circuit, if it is plane, or the 

 maximum projection of its surface on a plane, which we will call the 

 plane of the current. When such a circuit is movable in a uniform 

 magnetic field, like that of the earth, stable equilibrium corresponds 

 to the case in which the flow of force across the negative face of the 

 circuit is a maximum ; the plane of the circuit tends then to set at 

 right angles to the force. Under the influence of the earth this plane 

 will be perpendicular to the dipping needle ; the current will move 

 from east to west in the lower part. If F is the strength of the field, 

 I that of the current, the potential energy of the current in the 

 position of equilibrium is 



Wj = - ISF ; 

 when the face of the current is turned upside down, it becomes 



W mF 

 o JLO J7 



