TOO 



PHYSIOLOGICAL PHYSICS. 



[Chap.X. 



sensitive instrument, Laving as many as 30,000 coils 

 of fine wire. The form of instrument mainly used 

 now is the reflecting galvanometer, of which two 

 forms will be described, that of Sir William Thomson, 

 largely employed in this country, and one of a German 

 origin, called Wiedemann's galvanometer. 



The feature of Sir William Thomson's instrument 

 (Fig. 53) is the small size of the needles, so that they 

 possess little weight, with a high degree of magnetisa- 

 tion. The magnets are very thin, 

 usually not more than one- eighth 

 of an inch long, and are arranged 

 in two sets, an upper and a lower, 

 connected together by an aluminium 

 rod. The needles of each set are 

 arranged astatically. Round each 

 set is a separate coil of wire, the 

 lower coil (6, Fig. 53) having 

 its course in opposite direction to 

 the upper. The coils are brought 

 very near to the needles, and con- 

 tain a very large number of turns 

 of fine wire. Fixed to the upper 

 set of needles is a slightly concave 

 Fig. 53. Sir Wm. mirror, not more than one quarter 



Thomson'sReflect- r> i v j_ rrn 



ing Galvanometer, of an inch in diameter. Hie 

 system of needles and mirror is so 

 light as to weigh barely a grain. 



The system is suspended by a single fibre of fine 

 silk from a brass pin fixed on the top of the vulcanite 

 frame of the coils. The coils are supported on brass 

 uprights. The whole apparatus stands on a vulcanite 

 disc, brass-bound, and levelled by three screws, and is 

 enclosed in a brass-bound glass shade. The cover of 

 this shade is of brass, and supports a brass rod c, on 

 which slides a large curved magnet d, feebly magnetised, 

 by which an artificial meridian can be created in any 



