BILATERAL DEFLECTION. 



289 



as A is greater or less than 0-5. In the former case the zero is 

 in a position of unstable equilibrium, and there are two adjacent 

 positions P and P' of stable equilibrium on each side. In the 

 second case the curves only intersect at the point O, and this 

 point is a position of stable equilibrium. 



The ratio A being less than 0-5, suppose that we displace the 

 curve sin fi by the angle a, which amounts to saying that the initial 

 position of the needle is not in the plane of symmetry, but at O' 

 (Fig. 175). When the current passes, the needle sets at rest at 



Fig. 174. 



Fig. 175- 



the point P, and this position is nearer 90 the greater was the 

 primitive divergence a. 



The condition of equilibrium 



a sin (/5 - a) = A sin 2/3 



shows that if the original deviation a is very weak, the angle P 

 and the displacement /3 a = 8 of the needle are both proportional 

 to the initial displacement. 



902. USE OF THE ELECTRODYNAMOMETER WITH ALTERNATING 

 CURRENTS. The electrodynamometer (853) lends itself especially 

 to the measurement of periodical alternate currents, provided the 

 period is small in comparison with the time of vibration of the 

 movable coil, and that the current passes entirely in the two coils. 



VOL. II. U 



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