8 SMALL WATER SUPPLIES. 



hand-driven dynamo E ; so that by turning the handle 

 W of the dynamo, a current traverses the meter and 

 the water in the conductivity tube G. The pointer 

 of the meter is deflected, and comes to rest at some 

 point upon the scale which directly indicates the 

 conductivity of the water in the tube. The test is 

 completed as soon as the pointer has come to rest, 

 that is to say, in two or three seconds. 



The tube G is made long enough, and the elect- 

 rodes are given sufficient surface, to make the electric 

 resistance in the paths of the current-path immedi- 

 ately surrounding the electrodes negligibly small, 

 compared with that of the length of water in the 

 tube. Hence gas bubbles may accumulate on the 

 electrodes without making any observable difference 

 in the measured conductivity. Moreover, gas bubbles 

 liberated from the electrodes rise upwards and escape 

 freely at the upper ends of the tube ; they can never 

 diminish the conductivity by travelling downwards 

 into the path of the electric current. The electrodes 

 are short, hollow cylinders of platinum, so that they 

 present a large surface, from every part of which gas 

 bubbles are free to escape upwards. 



Water derived from springs or wells in the chalk 

 is inevitably a hard water. Where there is no 

 sewage pollution the conductivity is an accurate 

 measure of the hardness, and the new method takes 

 the place of the " soap test," with all the advantages 

 of rapidity, precision and simplicity. The hardness 

 may be due either to carbonate of lime or carbonate 

 of magnesia, or both ; and if it is necessary to dis- 

 criminate, a chemical analysis must be made once 

 for all. After that, periodic tests of conductivity 



