MEASUEING ELECTRICAL CONDUCTIVITY 27 



and allowed to drain^ for a definite time (not over 

 one minute) after which the resistance becomes practi- 

 cally constant. 



The current passes for a short distance through sea 

 water before reaching the disks. There is a film of sea 

 water between each pair of disks and likewise a film 

 around the cut edges. Otherwise the current passes only 

 through the tissue. 



As soon as the resistance has been measured the 

 apparatus is replaced in sea water ; the set screw, N, is 

 loosened so that the electrode holders can be moved 

 apart and the disks separated from each other by means 

 of forceps. After standing for a few minutes in sea 

 water the resistance is again determined. The disks 

 are then separated as before and allowed to stand in sea 

 water. This procedure is continued until it becomes 

 evident that the resistance is practically stationary.^ 



The apparatus is then transferred to another solu- 

 tion (e. g., NaCl 0.52 M) having the same conductivity 

 (and temperature^") as the sea water. There should 

 be at least 1,500 cc. of solution, contained in a shallow 

 dish of glass or enameled ware. Th6 disks are at once 

 separated by means of forceps and thoroughly rinsed 



' Each support rests on a block of paraffin. Care must be taken that 

 there is no conduction between the blocks; e.g., along the wet surface of 

 the table. 



'Unless this is the case the material is rejected. With good material 

 the resistance remains stationary for a long time; in one experiment it 

 remained so for 10 days at about 20° O. In this case the tissue was 

 kept in running sea water and was only half-submerged, thus ensuring 

 an abundant supply of oxygen. See Osterhout (1915, B). When placed 

 on ice Lammaria can be kept in good condition for a muiih longer time. 



"All readings should be made at the same temperature or, if this is 

 not practicable, should be corrected to the standard temperature. For the 

 temperature coefficient, see p. 37. 



