52O MEASUREMENT OF RESISTANCES IN ABSOLUTE VALUE. 



The oscillations of the frame itself in a magnetic field, with an 

 open or a closed circuit, will furnish an analogous expression. 



Let us suppose that the frame of surface S is suspended by a wire 

 whose couple of torsion is C, and is in equilibrium in the meridian ; 

 the flow of force which traverses it for a deflection x is HS sin x, and 



the corresponding electromotive force HS cos x . The equation 



at 



of the current is 



dl dx 



L + RI + HSCOS* = 0. 

 dt dt 



The moment of the couple which acts on the current is 



For very small deflections, the equations of the current and of 

 the motion reduce to 





If these equations are compared with equations (12)' and (14)' 

 of 845, it will be seen that it is sufficient if we replace MG by HS, 

 and HM by C, the other letters retaining their signification. 



We shall have then, for the resistance of the circuit, 



(6) 



1107. ALTERNATING CURRENTS. The continuous rotation of 

 a frame in a magnetic field, or of a magnet in a frame, produces a 

 periodical electromotive force; and when the permanent regime is 

 established, the current is itself periodic and alternating. 



