104 PHYSIOLOGICAL PHYSICS. [Chap. x. 



screw a wire is led to each of the outer binding screws 

 of the galvanometer. Between the two binding 

 screws of the shunt is a hole, and if the plug be 

 inserted here as in the figure, the current is short- 

 circuited ; for the current merely travels across from 

 one screw to the other and back to 

 the place whence it came, none going 

 to the galvanometer, because of the 

 greater resistance. If this plug be 

 now removed, as well as the other shown 

 in the figure, the current reaching one 

 screw cannot get straight across to the 



Fig 55^Gaiva- ^ ner ) but must traverse the galva- 

 nometer Shunt, nometer. Suppose, however, one of 

 the plugs be inserted in the hole 

 marked ^, then such resistance is interposed in the 

 short circuit that -j^th of the total current goes to 

 the galvanometer, and the remaining T %ths are short- 

 circuited. If the plug be put into the hole marked -g^-, 

 only T^roth P ar t g es to the galvanometer, if into 

 -g-i-g- only T J^th part goes to the galvanometer. 



Each shunt is graduated for the instrument which 

 it accompanies. For the coils of the shunt must be 

 graduated according to the resistance of the particular 

 galvanometer, since it is the ratio between the resis- 

 tance of the galvanometer and that of the shunt that 

 determines what proportion of current will go to the 

 galvanometer, and what will be short-circuited. 



Wiedemann's galvanometer, or boussoSe 

 is shown in Fig. 56. It consists of a thick cylinder 

 of copper, through which a tunnel is bored. This 

 tunnel can be closed at each side by a cover with glass 

 front, or by a solid plug of copper. Within this copper 

 chamber hangs a magnetised ring A, shown at the side, 

 of such a size that it has just room to swing clear on 

 all sides. Connected with the ring is an aluminium 

 rod which passes up through a copper tube and is 



