MEASURING ELECTRICAL CONDUCTIVITY 23 



solution having a lower conductivity than the bacteria, 

 but not in a solution of the same conductivity. 



The method therefore consists in placing the bacteria 

 in an electric field and increasing the 

 strength of the salt solution until they 

 cease to orient. The results indicate 

 that the conductivity of living bac- 

 teria is usually greater than that of 

 the medium in which they grow. This 

 is opposed to the results of Shearer 

 (1919, ^) in which the conductivity 

 was measured in the usual manner. 

 On theoretical grounds there are ser- 

 ious objections to Thornton's tech- 

 nique as well as to his conclusions. 



The method' used by the writer 

 gives under the most favorable condi- 

 tions, measurements which are 

 accurate to within 1%. This degree 

 of accuracy may be regarded as satis- 

 factory for biological purposes. 



In the original method* aquatic 

 plants with leaf-like fronds were 

 employed, particularly one of the 

 common kelps of the Atlantic coast 

 (Laminaria agardhii) : disks were cut 

 from this by means of a cork borer 

 and packed together, like a roll of coins, 

 in an apparatus which is shown in Fig. 3. It consists 

 of two platinum electrodes (covered with platinum 

 black). A, sealed into glass tubes, B, which are filled 



'This was developed without reference to the methods previously 

 used and differs somewhat from them. 

 •Osterhout (1918, E). 



Fig. 2. — Apparatus for 

 mcaBuring tne electrical 

 conductivity of eea urchia 

 eggs (Gray) : S, silver wires, 

 (?, (?,_ glass tubes; E, plat- 

 inum electrodes. 



