On the tilted electrometer

A detailed description is given of the construction and performance of a new, evacuated, critically damped, quick-reading, quartz-fibre electrometer. The sensitivity obtainable is limited only by the Brownian motion of the fibre. Photographic records are reproduced showing the response of the electrometer to suddenly applied potential-changes, and to the ionization currents of individual α particles. The minimum potential-change that can be measured (with the usual convention that the corresponding deflection of the system be not less than four times the root of mean square of the deflections of the Brownian motion) is of the order of 0.0001 V. when the period (undamped) is 5 seconds, and 0.0005 V. when the period is 1 second. The range of approximately constant sensitivity is adequate for most purposes.