OF ELECTRO-DYNAMIC FORCES. 491 



they obtain in the instrument. Hence also the common diameter 

 of the two coils, or their axis of rotation, has a vertical position, 

 in order that the rotation may be performed in a horizontal plane. 



That coil which is to be rotated, to allow of the onward trans- 

 mission and retm-n of the current, must be brought into con- 

 nexion with two immoveable conductors ; and the main object 

 of the instrument is to effect these combinations in such a 

 manner that the rotation of the coil is not in the least interfered 

 with even when the impulse is the least possible, as occurs when 

 these connexions are effected by means of two points dipping 

 into two metallic cups filled with mercury in which the two im- 

 moveable conductors terminate, as in Ampere's arrangement. 

 Instead of these combinations, which on account of the unavoid- 

 able friction do not allow of the free rotation of the coil, in the 

 present arrangement two long and thin connecting wires are 

 used, which are fastened at their upper extremities to two fixed 

 metallic hooks, in which the two immoveable conductors termi- 

 nate, and at their lower extremities to the frame of the coil, and 

 are there firmly united to the ends of the wires of the coil. The 

 coil hangs freely suspended by these two connecting wires, and 

 each wire supports half the weight of the coil, whereby both wires 

 are rendered equally tense. 



These two connecting wires thus effect the transmission of the 

 galvanic current from oneof the immoveableconductors to thecoil, 

 and back to the other immoveable conductor ; and they effect this 

 without the least friction interfering with the rotation of the coil. 



These connecting wires are also of service, because each 

 rotation of the coil through a certain angle corresponds to 

 a definite rotatory momentum, which tends to diminish this 

 angle, and is proportional to the sine of the angle of rotation ; 

 whence a standard is formed for all rotatory momenta, by the aid 

 of which any other rotatory momentum acting upon the coil 

 may be measured. This is effected according to those simple 

 laws which Gauss has developed in the case of the bifilar mag- 

 netometer. Lastly, this measure may be made more or less 

 delicate at pleasure, or as occasion may require, by the approxi- 

 mation or separation of the two connecting wires. This method 

 of suspension not being accompanied with any friction, allows of 

 increase in the weight of the suspended coil, which inay be any 

 iinount provided it is not more than the connecting wires are 

 .ipal)le of supporting. Hence a very long wire may be wound 



2 1. 2 



