In this gauge, comparable in size and simplicity to a Bayard-Alpert gauge, an electron beam (-1 OOv) of high perveance is fired into a "field free" drift space. Initially, the strong space-charge forces cause the beam to diverge radially. Positive ions formed by electron-gas collisions are trapped in the deep potential well of the beam and neutralize the space-charge so the beam gradually converges. The time-rate of convergence, proportional to gas pressure, is easily measured without external amplification as a current increase (of several milliamps) onto a target.
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