148 



ELECTRICAL MEASUREMENTS 



leads so that the frame may be raised or lowered by means of a 

 tackle and the immersion of the electrodes varied. 



If the conductivity of the water is known, the rheostat may 

 be designed to absorb a given amount of power. The arrange- 

 ment of electrodes is shown in Fig. 75. 



FIG. 74. Three-phase power-absorbing rheostat with cylindrical electrodes. 



The electrostatic capacity of two parallel cylinders in air, 

 diameter D cm., spaced a cm. on centers, length I cm. is * 



I 



C = 



4 log. 



a + 



D 



/ 



/ \ 



Hence, the conductance between 

 these cylinders when immersed in an 

 infinite medium of conductivity p' will 

 be 



1(3 QI - 



FIG. 75. Arrangement of 

 electrodes in three-phase 

 water rheostat. Let 



log, 



then 



9 = 



1.36 P 'Z 



o {# + \/K 2 - l} 

 * RUSSELL," "Alternating 'Currents," vol. 1, p. 102. 





