40 



PHYSICAL CHEMISTRY 



clamped on the stage of a tilted back microscope. If much cur- 

 rent is run through the electrometer, a drop of mercury must be 

 allowed to pass from the capillary into the large arm. 



The telegraph key short circuits the electrometer when not in 

 use, and if a simple key is used instead, a short circuiting switch 

 must be added. 



Fig. io. Weston standard cell, saturated type (from MRR). 



It is not usually economical to make a Weston cell (Fig. io) 

 if only one is needed and the materials are not already at hand. 

 Directions for making it are given in Ostwald-Luther, "Messun- 

 gen," 1910. Its voltage should be 1.0183 at 20 . If it is short 

 circuited it will not recover for several days. 



The potentiometer should have a high electrical resistance in 

 order to prevent the storage cell from running down rapidly, 

 and to reduce errors due to contact resistances within itself. 



If a standard potentiometer or one like Fig. 11 is not available 

 a No. 36 nichrome wire stretched over a 1018.3 millimeter scale 

 may be used, but it must be calibrated as the meter wire is in 

 Ostwald-Luther, "Messungen," 1910. 



