PHYSIOLOGICAL PHYSICS. 



[Chap. X. 



the action of terrestrial magnetism upon the needle. 

 In Nobili's arrangement, two needles ab a'b are taken, 

 both as nearly as possible equally magnetised. They 

 are united by a light piece of tortoise-shell, and are 

 ^^^ so placed that the north pole of one 



J | is opposite to the south pole of the 

 other (Fig. 51). 



If both needles have exactly the 

 same degree of magnetisation, then 

 the influence of the earth on the north 

 pole of one is neutralised by the pre- 

 cisely equal influence on the south 

 pole of the other. The result is that 

 with such a system of needles the 



Fig. 51. The As- v ,. r J.T xi > 



tatic Needle-pair, directive torce or the earths mag- 

 netism is removed, and then the 

 needles set perpendicular to the magnetic meri- 

 dian. Such a system is called ASTATIC. If such 

 a system be surrounded by a coil of wire (Fig. 52) 

 so that the under needle ab is in the centre of the 

 coil, and the upper needle a'b' 

 just above the coil, then a 

 current passed round the coil 

 will deflect both needles in the 

 same direction, according to 

 Ampere's rule. Thus, such a 

 system, being rid of the earth's 

 action, is not only free to obey 

 any other force, but by the 

 double needle the effect of a cur- 

 rent round the coil is increased. 

 Such an arrangement is conse- 

 quently able to detect a very much feebler current 

 than a single needle can. Should both needles not be 

 equally magnetised, then the earth will influence the 

 needle of greater magnetisation, and the system will 

 be brought into the plane of the magnetic meridian. 



Fig. 52. 



