746 ASTATIC NEEDLES. 



action upon them, because the force exerted on one 

 needle would be neutralised by the equal opposite force 

 exerted on the other, and the system would conse- 

 quently rest indifferently in any direction. But such 

 an equality of both needles cannot be obtained, one is 

 always a little more strongly magnetised than the other ; 

 hence the system obeys a directive force which is equal 

 to the difference of the directive forces exerted upon the 

 separate needles, and is therefore extremely small. The 

 consequence of this is that a feeble current passed either 

 above the upper needle or beneath the lower one, is 

 sufficient to deflect the system considerably. The 

 deflection is still greater if the current is made to pass 

 between the needles, for then the action of the current 

 upon the system is increased, both needles being 

 urged to turn in the same direction, as follows from the 

 rule stated previously. 



An astatic needle may be made of two pieces of a knitting needle 

 of equal length, which are magnetised by drawing the poles of a 

 magnet along each needle in the way previously described, doing it 

 the same number of times for each needle ; they are then connected 

 by means of thin copper wire as shown in fig. 376. The system is 

 suspended by a silk fibre, or by a fine silk thread. When the mag- 

 netism of both needles is nearly equal, and they are not fixed exactly 

 parallel, the system assumes a position which somewhat differs from 

 the correct approximate north-south position, but this does not 

 interfere at all with the experiments on the deflection. The wire 

 through which the current passes cannot be placed exactly between 

 the two needles so as to be in the same plane with them, as the wire 

 which connects them is in the way ; the current should be brought 

 within 1 or 2 mm from the wire. One Bunsen or Grove element is 

 sufficient to produce a distinct deflection of a common magnetic 

 needle ; for an astatic needle the current of one Meidinger cell is 

 amply sufficient. 



By means of astatic needles the presence of ex- 

 ceedingly feeble currents may be detected, if the con- 



