The Dependence of Interelectrode Capacitance on Shielding

Although it has been known for many years that the interelectrode capacitances of a vacuum tube with a glass envelope depend upon the shielding of the tube, no quantitative information on the extent and character of this dependence has been published. In the present work the general theory is developed with the help of Maxwell's coefficients of capacity and induction. A simple mathematical transformation shows why these cofficients may be used for practical measurements in which all potentials are measured with respect to the earth rather than infinity. The relation between the Maxwell coefficients and the interelectrode and electrode-to-earth capacitances is also developed. The effect of changes in shielding is not necessarily small. The experiments reported show that C
pf
for the particular triode studied can be made to vary from 0.16 micromicrofarad to 0.41 micromicrofarad by changes in the geometry of the external environment. Furthermore, ungrounding the shield with which C
pf
was 0.16 micromicrofarad made the "effective" C
pf
1.40 micromicrofarad.