3 20 



COMPARISON OF RESISTANCES. 



ends of which terminate in two lateral binding screws v and #', 

 the total resistance is 10000 ohms. 



Special plugs furnished with binding screws, and which are 

 placed in holes in the segments, render it possible to remove any 

 resistance from the set of dials. 



The wire used in resistance coils is usually very fine ; it would 

 be dangerous to pass through it powerful currents, which would 

 endanger the insulating substance, or even burn the metal. .Less 

 delicate resistances are obtained with rods of carbon, such as are 

 used for the electric light. These carbon resistances vary very little 



Fig. 181. 



with the temperature ; the connections are effected by means of 

 copper mounts. 



Very large and practically convenient resistances are obtained 

 by means of black lead lines ruled on ebonite, or better in a polished 

 groove in the ebonite. The ends of the line are connected with 

 binding screws, and the line itself is then covered with varnish.* 

 It is necessary to control these resistances from time to time ; 



* PHILLIPS. Phil. Mag., Vol. XL., p. 41. 1870. 



