354 



ELECTRICAL MEASUREMENTS 



tight, so a pressure gage must be supplied and the air pressure 

 renewed from time to time. 



Experience has shown that with careful construction the 

 pressure may not fall more than 20 Ib. during a year from an 

 initial value of 220 Ib. per square inch. 



Compressed gas condensers are used in connection with radio- 

 telegraphic apparatus. Fig. 209 shows one of those installed 

 at Arlington, Va., by the United States Government. 



Working Standards of Capacity. Though the determination 

 of the capacity and phase angle of working standards of capa- 





FIG. 210. Subdivided condenser. 



city depends ultimately upon the use of secondary air condensers, 

 such instruments are* not suitable for general use in the labora- 

 tory on account of their size, and more convenient arrangements 

 must be employed. Ordinarily the capacity of laboratory 

 standards is of the order of magnitude 1 microfarad. Such 

 standards should be subdivided and made adjustable by arrange- 

 ments for placing the various sections in parallel. When this is 

 done the net capacity in terms of the capacities of the various 

 sections is 



C = d + C 2 + C 3 + .. 



