1902] 



ELECTRICAL CONDUCTIVITY OF PLANT JUICES 



83 



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r 



polarization effects can be avoided by using an alternating cur- 

 rent of high frequency, and consequently a small induction coil 

 (/) of very rapid vibration is added, which serves to transform 

 the current derived from the batteries (B). 



Since the resistance of solutions varies within wide limits, it 

 was necessary to try a number of different kinds of cells before 

 one was found that was adapted to the work In hand. The first 

 form of cell tried is shown in Ji^. 2a. It has a diameter of 2^"^, 

 and the platinum electrodes [e) are fused into the ends of glass 



a 



b 



Fig. 2 — For explanation see text 



C 



tubes, which are filled with mercury, and securely set in the hard 

 rubber stopper so as to keep the electrodes in a fixed position. 

 The electrodes are covered with a layer of platinum-black by 

 electrolyzing a dilute solution of platinic chloride between them, 

 the object being to increase the surface of the electrodes and 

 thus minimize the polarization effects. This type of cell was 

 found objectionable for two reasons : first, on account of the 

 amount of juice necessary to cover the electrodes ; second, 

 because of the small resistance offered, thus making a very indis- 

 tinct minimum with the telephone. The second form of cell is 

 shown in jig. 2b. The diameter of the cell was S""", and conse- 

 quently the resistance of a small amount of solution could be 

 measured, but the minimum obtained with the telephone was 

 indistinct. The form of cell which proved most satisfactory is 



h^ 



I 



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