146 ELECTRICAL MEASUREMENTS 



grate about 15 in. in diameter is placed at the bottom of the 

 barrel and provided with a terminal of insulated wire. A second 

 grate, S', is screwed to an iron rod and suspended by a rope which 

 passes over pulleys to a counterweight. Short wooden pegs 

 prevent the two grates from being brought into contact. Fresh 

 water is used and the required conductivity obtained by adding 

 a salt, such as sodium carbonate. 



Such a "water barrel" will take about 70 amp. at 1400 volts, 

 absorbing about 100 kw.; the water will 

 boil violently when the rheostat is 

 forced to this extent. An adequate 

 water supply must be provided and 

 arrangements made by which the bar- 

 rel may be kept full without danger 



Compensating 

 Cell 



I'h 



FIG. 71. Water rheostat for small 

 currents. 



FIG. 72. Water-barrel 

 rheostat. 



to the operator. On account of the steam and gases, such rheo- 

 stats should be used out-of-doors. 



When rheostats of this general form are used, there is always 

 more or less slopping over of the water. The ground and sur- 

 rounding objects often become thoroughly saturated and the 

 greatest care must be exercised by the attendants that severe or 

 perhaps fatal shocks are not experienced through inadvertantly 

 touching some of the wiring. One must not relax his vigilance 

 because the voltage is low, for with sufficiently good contacts, 

 shocks from 110- volt circuits have proved fatal. The station 

 for the'^operator should be properly raised from the ground, 

 so that the platform will be dry and the ropes by which the elec- 

 trodes are manipulated should be rendered safe by the intro- 

 duction of strain insulators. 



