ELECTROPHYSIOLOGY 



351 



Loeb and Beutner. The complete apparatus for performing 

 the experiment is ilkistrated in Fig. 153, and the diagram given 

 in Fig. 154. 



Fig. iry.\. 



—Apparatus for determining the potential difference between the cut 

 and the whole surface of an apple. 



An apple is put in a bowl containing a salt solution (O.OlTlf KCl). 

 A depression cut in the top of the apple holds a similar salt 

 solution and an electrode (or agar bridge leading to an electrode). 

 The apple, with its solutions and electrodes, is inserted as the 

 unknown electromotive force in a potentiometric circuit, just 

 as in the case of an electrometric setup for determining pH 

 (Fig. 145). If an external electromotive force (storage cell) 

 is first balanced to a known potential, viz., that of a standard 



Fig. 154. — Diagram of the flow of current (i.e., of electrons) from the cut 

 (upper) surface of an apple containing a salt solution and an electrode, through 

 a resistance, a galvanometer, an electrode, a surrounding electrolytic solution, 

 to the whole (bottom) surface of the apple and through the apple. 



cell, and then to an unknown one, viz., that of the apple skin, 

 the unknown potential can be calculated (page 314). A potential 

 of some 40 mv. (0.040 volt) is thus obtained for the apple. 



