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TEXT-BOOK OF PHYSIOLOGY. 



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 evapora- 

 tion. The electricity is generated at the surface of the zinc cup by the 



chemic action of the chlorin which 

 arises from the dissociation of the am- 

 monium chlorid. When the plates 

 are united by a conjunctive wire the 

 current within the cell flows from the 

 zinc (the positive element) to the 

 carbon (the negative element), and 

 without the cell from the carbon (the 

 positive pole) to the zinc (the negative 

 pole). 



Leads. By means of insulated 

 wires attached to the poles of a cell, 

 the electricity may be conducted from 

 the cell and used for exciting or 

 stimulating purpose. As the wires 

 thus become practically prolongations 

 of the plates their ends become the 

 corresponding poles. In experimental 

 work the ends of the wires are provided 

 with special devices, termed 



Non-polarizable electrodes. 

 The necessity for the employment of 

 such electrodes arises from the fact 

 that when the ends of the wires from 

 a cell are placed in direct contact with 

 the tissues chemic changes are pro- 

 duced in a short time, which lead to their polarization. As a result, a 

 current opposite in direction to that of the cell is developed, which tends 

 to weaken or neutralize it. This polarization current vitiates the result 

 of many experiments made with highly irritable tissue such as nerve tissue. 

 Whether for stimulating purposes or for the purpose of detecting the exist- 

 ence of electric currents in living tissues, it is essential that the electrodes 

 used shall be non-polarizable. The earliest electrodes of this character 

 were made by du Bois-Reymond and were based on the fact discovered 

 by Regnault that a strip of chemically pure zinc or amalgamated zinc 

 (Matteucci) immersed in a saturated solution of zinc sulphate would not 

 polarize. One form made by du Bois-Reymond is shown in Fig. 337. It 



FIG. 337. NON-POLARIZABLE ELEC- 

 TRODES, i. Du Bois-Reymond's. 

 2. Von Fleischl's. 3. d'Arson- 

 val's. 



