PHYSIOLOGIC APPARATUS 



759 



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 over- 

 come these disadvantages, cells have been devised which are less violent in 

 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 saturated 

 solution of CuSO 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. 334). 

 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. 

 TJie 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 respectively 

 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 to the flow of 

 electricity by the fluids between the zinc and the copper. In all measurements, 

 the internal resistance of the cell must be taken into consideration. 



The Dry Cell. The commercial dry cell is a convenient source of 

 electricity for general laboratory work. It consists of a cup of zinc, the inner 

 surface of which is covered over with a thick layer of a paste of plaster of 

 Paris, saturated with ammonium chlorid. In the center of the cup there is 

 a rod of carbon. Surrounding this rod and occupying the space between it 

 and the plaster of-Paris paste, is a mixture of manganese dioxid and charcoal. 

 The upper surface of the cell is sealed to prevent evaporation. The electric- 

 ity is generated at the surface of the zinc cup by the chemic action of the 

 chlorin which arises from the dissociation of the ammonium chlorid. When 



FIG. 334. DANIELL CELL. 



