MEASURING ELECTRICAL CONDUCTIVITY 33 



rial) and an electrode carrier at each end are placed in 

 a V-shaped trough with rigid ends ; at one end is a screw 

 by means of which they can be forced together and held 

 with any desired degree of pressure. At the places 

 where the pieces of material are located, the trough is 

 cut away so that they do not come in contact with it. 

 Care is taken to keep the current from leaking along the 

 trough (its surface is covered with paraffin). 



The current therefore flows through the glass cells 

 and through the pieces of material placed between them. 



The advantages of this type of apparatus are: (1) 

 the end pieces do not have more resistance than those 

 in the middle; (2) the solutions may be changed without 

 disturbing the material. 



Types A and B may be combined by substituting 

 disks of Laminaria for the glass cells. 



Type C is shown in Fig. 8. It consists of two elec- 

 trode carriers similar to those in Type A. The material 

 is shown at M, its edges being completely surrounded 

 by vaseline, F, F, so that the current cannot leak out. 

 In many cases it is preferable to use chicle, grafting wax, 

 or art gum in place of vaseline. The apparatus remains 

 partly submerged (the water line being indicated at W, 

 TF), thus keeping the temperature more nearly constant. 

 The solutions are changed by siphoning through the 

 openings which admit the electrode tubes. This makes 

 it unnecessary to unscrew and separate the electrode 

 carriers during the experiment. 



Type D is shown in Fig. 9. It permits the use of in- 

 tact plants. One end of the plant is inserted in each 

 of the cells A and B and held in place by a split rubber 

 stopper. The cells A and B are filled with solution. 

 The free portion of the plant is bathed in any desired 



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