CHAP, in.] ELECTRICITY AND MAGNETISM. 



155 



If two magnetised needles of equal strength and size are 



bound together by a light wire of brass, or aluminium, 



in reversed positions, as 



shown in Fig. 81, the force 



urging one to set itself in 



the magnetic meridian is 



exactly counterbalanced by 



the force that acts on the ^-^r: 



other. Consequently this 



pair of needles will remain 



in any position in which it is 



set, and is independent of the = 



Oi 

 Fig. 81. 



earth's magnetism. Such a 

 combination is known as an 

 astatic pair. It is, however, difficult in practice to 

 obtain a perfectly astatic pair, since it is not easy to 

 magnetise two needles exactly to equal strength, nor is 

 it easy to fix them perfectly parallel to one another. 

 Such an astatic pair is, however, 

 readily deflected by a current flowing 

 in a wire coiled around one of the 

 needles ; for, as shown in Fig. 82, 

 the current which flows above one 

 needle and below the other will urge 

 both in the same direction, because 

 they are already in reversed positions. 

 It is even possible to go farther, and 

 to carry the wire round both needles, 

 winding the coil around the upper in 



Fig. 82. 



the opposite sense to that in which the coil is wound 

 round the lower needle. 



Nobili applied the astatic arrangement of needles to 

 the multiplying coils of Schweigger, and thus constructed 

 a very sensitive instrument, the Astatic Galvanometer, 

 Shown in Fig. 88. The special forms of galvanometer 

 adapted for the measurement of currents are described 

 in the next Lesson. 



