244 



THE ELECTRIC TELEGRAPH. 



An astatic pair of needles never takes the direction of 

 the magnetic meridian, but assumes a position at an angle 

 which increases as the difference between the force of 

 the needles is diminished, or as they become more astatic. 

 Dubois calls this the arbitrary deflection of the needle pair. 

 The cause of this arbitrary deflection has been ascertained 

 by Nobili to be that the two needles are never suspended 

 absolutely in the same vertical plane, but that the vertical 

 plane which coincides with one needle makes always an angle 

 with that which coincides with the other. This will be 



easily understood from Fig. 126, 

 where n s and n' s' represent the 

 horizontal projection of two needles 

 of equal size and magnetic moment, 

 and N s the line of the magnetic 

 north and south. 



Suppose the force of all the 

 magnetic elements of the needles 

 which tend to turn them to the 

 poles N and s to be collected in the 

 ends n, ri, s, and s', and to act 

 in lines parallel to the line N s, 

 then, if / represent the force acting on the end n t the 

 product / I will be the static moment with which this 

 force tends to turn the north end of this needle in the 

 direction of the arrow, and 2 f I that exerted upon the 

 whole needle, the south pole being attracted in the reverse 

 direction, but in the same sense with regard* to the point 

 of suspension o. The other needle, n' tf, having an equal 

 amount of magnetism, but being at an angle a with n s, 

 the total force with which it is drawn round in the other 

 direction is 2/ 1'. 



It is evident, therefore, that the needle system can only 

 come to rest when the opposing forces 



or when 



2 f 1=2 ft, 

 1= I 



