n6 PHYSIOLOGICAL PHYSICS. [Chap. XL 



the deriving cushions from one trough to another, it 

 may be seen whether they are at fault. As a rule, the 

 error will be found to exist in the amalgamation of 

 the troughs, or in their outer varnishing, which may 

 have cracked somewhere so as to expose un amalga- 

 mated zinc, or in want of saturation of the zinc 

 solution. The sources of error being removed by the 

 evident remedy, the deflection will disappear. Often 

 a very slight deflection may be caused to vanish by 

 setting up the troughs for twenty-four hours before 

 they are required and letting them stand for that 

 time, connected by means of the closing cushion and a 

 short piece of thick copper wire passing between the two 

 binding screws. To prevent evaporation, the troughs 

 should be covered with a glass shade, the inside of 

 which has a few pieces of wet blotting paper adhering 

 to it. It is not necessary to make fresh cushions on 

 every occasion the troughs are used. If they have 

 been properly stitched they may be placed in a well- 

 stoppered bottle with sufficient zinc solution to cover 

 them. If they are regularly returned to the bottle 

 after being used, they may keep for years. If they 

 have been permitted to get encrusted with zinc salts 

 by evaporation, suspend them for twenty-four hours in 

 distilled water. At the end of that time gently 

 express the water, and then place them for a few 

 hours in a saturated solution of the zinc sulphate, 

 after which they will be again ready for use. 



To determine the direction of the current 

 sent through the galvanometer a preliminary experi- 

 ment is necessary. Let the galvanometer be con- 

 nected with a key in short circuit ; to one wire from 

 this key attach a slip of zinc, and to the other a slip 

 of copper. Place the slips in a glass with dilute sulphuric 

 acid, so that they just dip into the fluid. From this 

 small element send a momentary current to the 

 galvanometer by quickly opening and closing the key. 



