Image Number 3 for United States Patent #4687964.
A field emission cathode 12 is positioned in a pair of intersecting cross grooves 30, 32 in the end of a ceramic tube 14 by a metal end cap 24. A spring 34 in electrical contact with the base of the cathode provides the necessary pressure to maintain continuous circumferential electrical contact between the gate film and a raised edge 38 on the end cap. With this structure the cathode chip is self-centering and easily replaceable. Also the gate film of the cathode is not abraded or rubbed during installation, and the holder is readily degassed.