21 POWER DEVICES-NAMELY, IGNITRONS, RECTIFIERS, THYRATRONS; TRANSMITTING TUBES-NAMELY, TRIODES AND TETRODES; RECEIVING TUBES; VACUUM CAPACITORS; MICROWAVE TUBES; ELECTRO-OPTICAL DEVICES-NAMELY, TELEVISION CAMERA TUBES, VIDICON PHOTOSURFACES, IMAGE INTENSIFIERS, SHUTTER TUBES, STORAGE TUBES, GLOW MODULATORS AND FLASH TUBES; EXPLODING BRIDGE WIRE DEVICES-NAMELY, DETECTORS, SQUIBS AND TRIGGERED VACUUM GAPS; SPARK GAPS; OZOTRONS; METHANE DETECTOR ELEMENTS