MOVABLE COILS. 235 



equilibrium ; this second method is generally employed. We have 

 then, for a bifilar suspension, 



L^ 



HSI cos8 = C sin 8, 



or 



and for a unifilar suspension, 



HS cos 8 ' 



This method is due to Weber.* If the suspension is bifilar, the 

 two wires are used to convey the current to the coil ; if it is unifilar, 

 the circuit is completed by a second vertical wire above the coil, and 

 attached to a slight spring, or by a rod which dips in a mercury cup. 



In order to regulate the original position of the coil, its axis is 

 put almost in the meridian, and the suspension is adjusted until the 

 passage of a current in the coil produces no deflection ; the suspen- 

 sion d\?the whole apparatus is then turned through 90. 



The slight defect of adjustment which might still remain is cor- 

 rected by reversing the direction of the current in the coil. If a is 

 the angle of the axis of the coil, with the perpendicular to the 

 meridian, 8j and 8 the deflections on each side for the two directions 

 of the current, with a bifilar suspension, the equations of equilibrium 



HSI cos (8 + a) = C sin 8, 

 HSI cos(S'-a) = C sin 8', 



would enable us, as has been seen above (834), to calculate the 

 angle a; but if the deflections 8 and 8' do not differ much, the 

 angle a which represents the error of adjustment is itself very small, 

 and we have sensibly 



C 8 + 8' 



I = tan . 



HS 2 



* W. WEBER. Electrodyn. Maasbestjnimungen, Vol. I., p. 16. 1846. 



