670 



Mr. R. A. Lehfeldt on a Potentiometer 



The sliding contact is carried by a block sliding on two steel 

 bars : it is moved by a steel rod with clamping and fine 

 adjustment screws, and its position read through a lens by 

 means of a vernier graduated in fortieths of a millimetre. 



Fig-. 2. 



As the wire is 100 millims. long the smallest reading is 4 ^ 

 of the unit, which, as stated above, may be 1000, 100, or 

 10 microvolts ; and as there are 20 tenth-ohm coils, the 

 smallest reading is —^ of the largest. 



The fineness of reading is, as a matter of fact, limited by 

 the sensitiveness of the galvanometer used. 



All the working parts are inclosed by a plate-glass lid 

 through which only the handles project. 



Outside the potentiometer itself the apparatus used consists ot 

 the accumulator (sometimes two or more) ; the adjustable re- 

 sistance R ; the cadmium cell ; galvanometer — a highly sen- 

 sitive D'Arsonval of about 20 ohms, made by the Cambridge 

 Scientific Instrument Co. ; and galvanometer key. The latter 

 is of the usual short-circuiting reversing type, but unusually 

 small, made of copper, and inclosed in a box from which 



