Instrumentation and measurements

Small variable three-terminal air capacitors are becoming of increasing importance. Bowdler¹ has constructed such a capacitor whose capacitance is known to 1% over the range from 0.01 to 10µ µf. This wide range is obtained by retracting the guarded electrode into a hole in the guard electrode. This produces an exponential relation between capacitance and motion of the electrode. It is similar to a capacitor previously described by Moon and Sparks⁵⁸ and recently mathematically formulated, by Snow⁵⁹.