GB251307A - Improvements in or relating to lamps for use in producing photographic records of electrical variations - Google Patents

Abstract

251,307. Case, T. W. Oct. 20, 1924. Vacuum tubes.-Discharge lamps for producing photographic records of electric variations produced by sound variations or other means comprise two closely associated electrodes in an ionizable gas such as helium, at least one electrode comprising oppositely disposed portions coated with directly applied alkaline earth oxide and used thus coated as the cathode and not used for coating another electrode as described in Specifications 220,040 and 220,041. In photographing sound variations, the microphone current, amplified one or more times by thermionic valves, is passed through the primary of a transformer the secondary of which is in series with a, source of potential and with the lamp, a moving photographic film being exposed to the cathode glow through a narrow slot. The cathode may comprise a U-shaped platinum filament 16 coated with barium or strontium oxide and the anode a flat plate 15 or a second U-shaped coated filament; in the latter case, the anode may be used as cathode when the cathode coating has been dissipated. Electrons emitted from one portion of the cathode strike the opposite portion, the fluorescent and photo-electric properties of the coating producing a concentrated cathode glow separate from the anode glow. The pressure in the bulb is 20 to 300 mm. of mercury and preferably 50 to 60 mm., and the distance between the anode and cathode is one-sixteenth to one-quarter of an inch and preferably one-eighth of an inch, the lamp operating at low potentials such as 150 volts. In forming the lamp, the platinum strip for the cathode 16 is coated with calcium nitrate and then with a compound of an alkaline earth metal such as barium nitrate, the former preventing reaction of the latter on the strip. The strip is heated in air to change' the nitrates to oxides, the electrodes are assembled in the glass or quartz bulb, the strip 16 is heated by current to drive off moisture from the coating, and the bulb is heated in a furnace to 540‹ C. with a partial vacuum maintained. After cooling, the bulb is exhausted and the plate 15 freed from gas by electronic bombardment on passing a large current through the strip 16, helium then being introduced until the required glow is obtained with a potential source and resistance in series with the tube.