348 



ELECTRICAL MEASUREMENTS 



of the spherical condenser used by Rowland in the determina- 

 tion of v in 1879, by Rosa in 1889, and by Rosa and Dorsey in 

 1905, is shown in Fig. 206. 



The capacity in electrostatic units of such a spherical air con- 

 denser is 



where R and r are the radii bounding the dielectric. 



H 



FIG. 206. Section of spherical air condenser. 



The internal radius of the spherical shell in the Rowland con- 

 denser is 12.67158 cm., and the radius of the ball is 10.11806 cm. 

 (at 20). The capacity is, therefore, 50.2095 electrostatic units. 



When the condenser is used, the ball must be carefully centered 

 and corrections made for the holes in the shell, the bushings, and 

 the cord by which the ball is suspended. The error in the final 

 calculated value of the capacity was estimated at about 2 parts in 

 100,000. When measured by Maxwell's method (see page 362) 

 the capacity was found to be 5.59328 X lO" 20 absolute electro- 

 magnetic units or 0.0000559328 microfarad. 



