MEMBRANE POTENTIALS 



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with the gelatin chloride solution are dipped into beakers con- 

 taining 350 c.c. of aqueous HC1 solution of originally the same 

 pH as that of the gelatin chloride solution, but free from gelatin. 

 The experiments last 20 hours or more at 24C. to allow the 

 establishment of osmotic equilibrium between the two solutions 

 (which requires only about 6 hours under the conditions of the 

 experiments). After 20 hours or more the P.D. between the 

 gelatin solution (which we call the inside solution) and the aqueous 



FIG. 40. Method of measuring the P.D. between gelatin chloride solution in a 

 collodion bag and the outside HC1 solution in beaker. 



solution (which we call the outside solution) is measured with the 

 aid of a Compton electrometer, giving a deviation of about 2 

 mm. on the scale for 1 millivolt at a distance of about 2 m. The 

 two electrodes leading to the electrometer are identical (Fig. 40). 

 They are calomel electrodes filled with saturated KC1 solution. 

 One electrode dips through a capillary glass tube into the gelatin 

 solution, the other also through a capillary glass tube into the 

 outside solution. In order to allow the electrode to dip into the 

 gelatin solution, the glass tube serving as a manometer is replaced 

 by a funnel, as shown in the figure. In the figure the upper level 



