The modulation method is a good method to measure the pressure in the region near the measurable lowest limit of B-A gauge. To apply this method on a specially prepared three grid modulated B-A gauge, the modulation characteristic curves of the gauge were studied. The curves were smooth in rather high pressure region, however, in extremely high vacuum region, sometimes curious zig-zags apeared, and the modulation method could not applied successfully.
It was clarified that these zig-zags were caused by change of X-ray flux irradiating on the ion collector and that the flux change was induced by the change in trajectories of electrons emitted from the filament. And it was confirmed that the zig-zags were severely appeared when the electron collector grid was constructed by straight thin wires, and not appeared when it was spiral.
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