| http://www.w3.org/ns/prov#value | - BRIEF DESCRIPTION OF THE DRAWINGS Reference will now be made to the accompanying drawings in which: FIG. I is a schematic view of an electron tube incorporating an illustrative embodiment of the present invcntion', FIG. 2 is a cross-section, on an enlarged scale, of the portion of the apparatus similar to that shown in FIG. 1, but incorporating an alternative embodiment of the present invention; a
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