ELECTROMAGNETIC RESISTANCE IS A VELOCITY. 503 



CHAPTER V. 

 PARTICULAR CASES OF INDUCTION. 



526. ELECTROMAGNETIC RESISTANCE is A VELOCITY. Consider 

 the case (Fig. 114) of a bar CC' sliding parallel to itself on two 

 parallel rails A A', BB', at the distance <, situate in a vertical plane 

 at right angles to the magnetic meridian, the ends of which are 

 connected by a metal conductor. Suppose that the direction of 

 the horizontal component of the terrestrial field is from back to 

 front. When the bridge CC' is moved away from AB, parallel to 

 itself, it is carried in the direction in which the action of the 

 Earth would urge it, if the circuit were traversed by a current 

 going from A to B by the bridge. This motion produces a 

 current of induction, which traverses the circuit in the opposite 

 direction that is, which goes from B to A by the bridge. 



If we consider that the resistance of the rails may be neglected 

 in comparison with that of the conductor which joins the points 

 A and B, and if R is the resistance of the circuit which we suppose 

 to be unchanged, x l and x 2 the values of the distance AC in the 

 two successive positions of the bridge, and H the horizontal com- 

 ponent of the terrestrial magnetic field, the corresponding quantity 

 M of induced electricity will be given by the equation 



RM = Q! - Q 2 = JH ( Xl - x 2 ) . 



As the product d(x l -x 2 ) represents the area S described by 

 the bridge, it follows that 



. 

 M 



In this expression the factor H is the intensity of a magnetic 

 field that is to say, a force which is exerted upon unit mass; 



