A Low Grid-Current Vacuum Tube

The various factors that may cause a current to flow to the control grid of a high vacuum tube are outlined. The magnitudes of the separate components are experimentally determined, and methods are given by which these currents may be greatly reduced. A tube is described which has a grid current of ${10}^{-15\mathrm{}}$ ampere and a mutual conductance of 25 microamperes per volt. As an input resistance of 4×${10}^{11}$ ohms may be used, a current of ${10}^{-17\mathrm{}}$ ampere may be detected when a galvanometer having a sensitivity of ${10}^{-10\mathrm{}}$ ampere per millimeter is used in the plate circuit. Under this condition the sensitivity is 250,000 millimeters per volt.