METHODS OF ELECTRICAL STIMULATION 47 



Bring the capillary into the field of the micro- 

 scope (Leitz objective 3, micrometer ocular), par- 

 allel to the micrometer scale. The end of the 

 tube should be just visible at the upper margin 

 of the field. If the meniscus is not visible, turn 

 the pressure screw slowly to the right until the 

 meniscus enters the field. Note the position of 

 the meniscus on the scale. Close the battery key. 

 Let an assistant place the metals in a beaker con- 

 taining solution of sodium chloride. Open the 

 short-circuiting key of the electrometer. 



When the metals touch the electrolyte a dif- 

 ference in potential will be set up, and the 

 meniscus will move in the capillary. 



Note the number of divisions of the scale 

 traversed by the meniscus. Close the electrom- 

 eter key. Wait several minutes. 



Now bring the meniscus back to its original 

 position on the scale. Open the electrometer key. 



The meniscus will move to a much slighter 

 extent than when the circuit was first made. 



adding 4 per cent of mercury to the molten zinc before casting ; 

 or the zinc may be dipped in 10 per cent sulphuric acid to clean 

 it, and mercury rubbed over the surface with a brush or a stick 

 padded with cloth ; or the zinc may be dipped in a solution 

 from which the mercury will deposit on the zinc. Formula for 

 amalgamating fluid : warm gently 4 parts mercury in 5 parts 

 concentrated nitric acid and 15 parts concentrated hydrochloric 

 acid until dissolved, and then add 20 parts more of concentrated 

 hydrochloric acid. 



