TANGENT GALVANOMETER. 



217 



as possible to the needle, and coil the wire so that the curve which 

 bounds the section of the channel satisfies equation U 2 = c 2 sm0. 

 It would be still better to distribute the wire in layers which satisfy 

 this condition, and in each of which the diameter of the wire 

 increases as the parameter c. 



Lastly, very small needles are used, so as to reduce to a mini- 

 mum the central cavity, which removes just those windings whose 

 special action is greatest. 



We may add that in the construction of the coil it is particularly 

 important to use very pure copper, and especially free from iron. 



Fig. 161. 



The presence of other elements rapidly diminishes the conductivity 

 of copper. Traces of iron give rise to local action, which become 

 considerable when the mass of the wire comes close to the needle. 

 The errors due to this cause were for a long time a source of serious 

 difficulties in using galvanometers of great sensitiveness. 



