The brownian motion of electrometers

Abstract

The Brownian motion is shown to be due for the greatest part to the irregular molecular collissions of the surrounding air, the thermal voltage fluctuations contributing only very slightly. From the expression

it follows that for a Hoffmann electrometer the root of mean square error at charge measurements, caused by the molecular collisions appears to be seventy times larger than the error arising from the thermal voltage fluctuations. Both the fluctuating torque exerted by the molecular collisions and the air damping, are aspects of the interaction of the instrument with the surrounding air. Air damping not being essential for the proper working of the electrometer, the Brownian motion may be reduced substantially by evacuating the instrument and applying an artificial damping which introduces almost no fluctuations. In a following paper †) the experimental verification of this method, reducing the Brownian energy by a factor of one hundred, will be described.