chap, x.] ASTATIC NEEDLES. 99 



But the influence of the earth will be very much 

 diminished, because it will only affect the system 

 according to the excess of magnetisation of the one 

 needle over the other. In point of fact it is difficult 

 to get a perfectly astatic system. Usually when the 

 system has been deflected by the action of a current, 

 on the removal of the current the needle will be found, 

 after a few oscillations, to come at last to rest in the 

 plane of the magnetic meridian. The more nearly 

 astatic the system, the slower will be these oscillations, 

 so that, by this means, one may test the condition of 

 the system. By the use of a feeble magnet, however, 

 a pair of needles not quite astatic may be made 

 absolutely so. It is only necessary to bring such a 

 magnet into the neighbourhood of the needle-pair, 

 and to keep it in the magnetic meridian, and with its 

 north pole pointing south. By bringing it gradually 

 nearer to the needle-pair a position is at length found 

 where it completely neutralises the earth's influence, 

 and perfects the degree of astaticism. By a similar 

 means a single needle may be made astatic. 



Aided by this astatic system of needles ISTobili 

 constructed a very sensitive galvanometer, by means 

 of which very feeble currents of electricity were 

 detected. The general form the galvanometer then 

 took was briefly this : A great length of fine copper 

 wire was wound on an ivory frame, each turn being 

 carefully insulated from its neighbour, and the ends 

 of the wire were connected with binding screws. The 

 needles were suspended from a support by a fine silk 

 fibre, so that one needle was within the coil, the other 

 just above it. The whole was carried on a block of 

 ebonite, and covered in by a glass case for the exclu- 

 sion of air currents. The chief modern workers who 

 have added to the sensitiveness of the galvanometer 

 are Du Bois-Revmond, of Berlin, and Sir William 



/ 



Thomson. The former himself constructed a very 



