Dating probably to the late Twenties or to the early Thirties, this Coolidge type bulbar line filament oil immersed tube is a single-focus tube, 10” (25cms) long with a 4” (10cms) bulb. It is fitted externally with a black painted spool-like aluminium heat sink fixed on the protruding anode shaft, and to which the high voltage is directly connected.

                    Note the slightly conical shape of the copper anode, and the unusual 30 degrees angle of the target surface. This angle falls half way between the 45 degrees  target angle in gas discharge tubes and early Coolidge type tubes, and the 10-15 degrees angle in the line focus tubes introduced in the Twenties.

                    No maker’s name, but the etching “Made in Holland” on the anode stem make me presume the tube was probably made by Philips.

                    A voltage adjusting rheostat is visible on the cathode end of the tube fitted to the external connections of the filament.