An automatically-controlled knudsen-type vacuum gauge

A vacuum gauge of the Knudsen1 type is described, by which pressures from 10¿8 mm to 10 mm of mercury are indicated on a moving-coil instrument used in conjunction with a range-change switch. The advantages and disadvantages of the Knudsen gauge have been listed elsewhere, the main criticisms being of its delicacy and slowness of response. The paper describes an instrument in which these disadvantages are eliminated and several additional advantages provided. The new gauge operates on the principle of balancing the pressure-dependent radiometric force by an electrostatic force, the pressure then being calculated from a measured potential difference and the dimensions of the gauge. The principal units of the instrument control system are described in detail and an overall analysis is developed.