PHYSIOLOGIC APPARATUS 



743 



2. The accumulation of hydrogen bubbles on the surface of the copper 

 linders the passage of the electricity. In a short time they develop a current 

 n the opposite direction, which also tends to weaken the original current. 

 Tiis action is termed polarization of the elements. 



Cells of this character are not suited for physiologic work, in which 



onstancy in the strength of the current is absolutely necessary. To over- 



ome these disadvantages, cells have been devised which are less violent i 



action which prevent polarization, and which maintain a current of constant 



strength for a long period of time. One of the most generally used for 



physiologic purposes is . 



The Daniell cell. This consists of a porous cup containing a satural 

 solution of CuS0 4 , copper sulphate/ in which is immersed a copper plate or 

 rod This combination is placed in a glass vessel containing a solution of 

 H 2 SO 4 (1:15). In this solution is immersed a roll of sheet zinc (Fig. 337). 

 Each of the plates is provided with 

 a binding screw. When the cell is in 

 action the sulphuric acid attacks the 

 zinc, forming zinc sulphate, and liber- 

 ates hydrogen; the cup being porous, 

 the hydrogen passes into the copper 

 sulphate solution, where it combines 

 with the sulphuric acid radicle, and 

 liberates metallic copper. Polariza- 

 tion of the copper is thus prevented. 

 The metallic copper is deposited on 

 the copper plate, which is thus kept 

 bright. The copper sulphate solution 

 is kept at the point of saturation by 

 packing around the copper cylinder a 

 quantity of the crystals of the salt. 

 The sulphuric acid passes back into 



FIG. 337. DANIELL CELL. 



EnterUresistance of the cell must be taken ^J^g*^ o 

 The Drv Cell. The commercial dry cell is a convenient source u 



When 



