PHYSIOLOGIC APPARATUS. 



673 



The cell as above described cannot maintain a current of constant 

 strength for any length of time, for the following reasons: 



i. The sulphuric acid solution, in consequence of its chemic action, 

 soon becomes nothing more than a saturated solution of zinc sulphate, 

 after which its chemic activity ceases. The current, therefore, soon 

 diminishes in strength. 



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

 hinders the passage of the electricity. In a short time they develop a 

 current in the opposite direction, which also tends to weaken the original 

 current. This action is termed polarization of the elements. 



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

 constancy in the strength of the current is absolutely necessary. To 

 overcome these disadvantages, cells have been devised which are less 

 violent in action, which prevent polarization, and which maintain a cur- 

 rent 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 con- 

 taining a saturated solution of 

 CuSO 4 , copper sulphate, in 

 which is immersed a copper 

 plate or rod. This combina- 

 tion 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. 336). Each of the plates 

 is provided with a binding 

 screw. When the cell is in 

 action the sulphuric acid at- 

 tacks the zinc, forming zinc 

 sulphate, and liberates hydro- 

 gen ; the cup being porous, the hydrogen passes into the copper sulphate 

 solution, where it combines with the sulphuric acid radicle, and liberates 

 metallic copper. Polarization 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 the porous cup, to take the 

 place of that used. This cell is remarkably constant for these reasons, 

 and well adapted for physiologic as well as other purposes where a current 

 of uniform strength is necessary. 



The projecting ends of the copper and zinc plates are termed respec- 

 tively the positive pole or anode, and the negative pole or kathode. The 

 electro-motive force of a Daniell cell is practically i volt; but when the 

 two poles are connected by a wire of i ohm resistance, the current strength 

 will be less than i ampere, possibly only 0.7, owing to the resistance offered 



43 



FIG. 336. DANIELL CELL. 



