244 MEASUREMENT OF CURRENTS. 



or 



For the same current the difference of pressure is proportional 

 to the intensity of the field, and inversely as the thickness of the 

 cavity. 



863. ELECTRODYNAMOMETERS. Weber's electrodynamometer* 

 is a movable coil, with unifilar or bifilar suspension, on which the 

 current of a fixed coil is made to act. In the original position of 

 equilibrium the two coils are at a right angle, and in order to 

 diminish the action of the earth as much as possible, the axis of 

 the movable coil is perpendicular to the magnetic meridian. 



If S' is the surface of the movable coil, I' the current which 

 passes through it, I the current of the fixed coil, these currents tend 

 to bring the axes of the coils near each other ; and for a deflection 8 

 from the original direction the couple of the reciprocal action is 

 II'GS' cos 8, the factor G being a mean value of the galvanometric 

 constant of the fixed coil in the space occupied by the movable coil. 



This couple may be counterbalanced by twisting the suspension 

 at the top through an angle 6, which brings the coil into the original 

 position. We have then, for a bifilar, 



II'GS' = C sin 0, 

 and for a unifilar, 



II'GS' = C0. 



The angle changes its sign when the direction of one of the 

 currents is reversed, but it remains the same when the direction of 

 both are reversed ; this is a characteristic property of the electro- 

 dynamometer. When the two currents are equal we have, according 

 to the case, 



* I2 = sin *- or v .e. 



If, on the contrary, the deflection 8 is observed, which the 

 movable coil takes up, the external field affects the condition of 

 equilibrium, and we have then, according to the mode of suspension, 



II'GS' cos 8 = (C S'I'H) sin 8, 

 II'GS' cos 8 = C8 S'I'H sin 8. 



* W. WEBER. Electrodyn. Maasbestimmungen, Part I. 1846. 



