The Capacitative Commutator

Abstract

The design and construction of a vibrating capacitor-type electrometer is described. Experimental data are presented to show that this electrometer reaches, within a small factor, the theoretical limit of sensitivity. The principle of the electrometer is a time-varying capacitor at the input of the device, which inverts the low frequency signal to a relatively high frequency, and raises its energy level, thereby simplifying the amplification of the voltage being measured, permitting the use of ordinary vacuum tubes, and producing an electrometer that is mechanically rugged and free from long period drift. Stabilization is obtained by negative feed-back. The electrometer was originally developed for radiation well-surveying in the petroleum industry, but has found other application in general radiation measurements.

Shot Noise in p-n Junction Frequency Converters*

“…This principle is used in vibrating-reed electrometers for amplifying very low frequencies.' , 2 To obtain microwave amplification, one needs a capacitance that varies at microwave or higher frequencies. Such a time-varying capacitance can be obtained by applying a microwave voltage to the voltagedependent capacitance of a p-n junction diode.…”

Mass Spectrometry

“…Consequently, new methods of detection or improvements in old ones are of immediate interest. One development that has been announced since the end of the war is the capacitativo commutator or dynamic condenser electrometer (119,120,130,131). This has extremely low drift, low background current, and high sensitivity.…”

An electrometer amplifier with two dynamic condensers

“…Several instruments re.ported m the literature operate on this principle [8][9]. A drawba~ of using one single dynami'c .condenser is that both th e signal and 1.f.components of the grid current noise of the .first tube are modulated.…”