Improvements in the Stability of the FP-54 Electrometer Tube

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Abstract
It was found that a considerable portion of the instability in the FP‐54 is caused by variations in emission from the thoriated tungsten filament. Operation of the filament at a current which neither activates nor deactivates was found to be a good criterion for adjusting the circuits employing these tubes. Large fluctuations were observed in emission immediately after activation of the filament. Greater stability was obtained by increasing the activation time from 8 to 40 minutes. Filament and shields improved the stability, showing that some rapid fluctuations in emission occur at the poorly activated end portions of the filaments. Long‐time drifts were not improved by end shields. Tubes with oxide‐coated filaments gave greater sensitivity and less grid current than tubes with thoriated tungsten filaments. These tubes, however, had a tendency to drift. A split type of FP‐54 was constructed with a common filament and space‐charge grid, but with twin control grids and plates. Both oxide‐coated and thoriated tungsten filaments were used. Operation of these tubes in a bridge type circuit eliminated long‐time drift and decreased the amplitude of rapid fluctuations.