A vibrating needle electrometer
Y L Yousef, R Kamel · Journal of Scientific Instruments · 1954
A dynamic type of mechanical electrometer, incorporating a simple electronic circuit, is described. The underlying principle is that the electrostatic force due to the charge under test is modulated by a small periodic component so that, instead of merely deflecting the suspended system, which in the present case is simply a metallic reed, it sets it also into resonant vibrations the amplitude of which is proportional to the charge, and may be easily measured directly on an output indicator. The electrometer can be used in a null arrangement in which the reed vibrations are prevented by an opposing voltage. The electrometer capacity is 3.9 pF, and the output indication per volt is 0.25 V readable to 0.005 V. A theory is given and the applications are discussed.