Patent GB-681164-A (GB)
Title: An improved system for regulating the positive electrode of an arc lamp
IPC: H05B31/00
Filed: 1949-06-27
Abstract: 681,164. Arc lamps. GRETENER. E. June 27, 1949, No. 16965/49. Class 39(i) [Also in Group XXXV] Apparatus for regulating the feed and position of the positive electrode of an arc lamp in which an image of the separation edge between the arcing end of the positive electrode and the arc and arc flame is projected on to a device sensitive to contrast in radiation of the projected image and which controls the feed mechanism, has an arrangement wherein the arc and arc flame are concentrated into the space in front of the positive crater by means of an air stream surrounding the positive electrode co-axially and issuing from the electrode holder to converge towards the arcing end of the electrode to increase the contrast and produce a substantially vertical front of the arcing end of the electrode. In the construction shown in Fig. 3 the positive carbon 1 is supported by a head 11 provided with a hollow space 12 into which is blown at 13 compressed air which issues out of holes 14 and concentrates the arc and arc flame in front of the positive crater 4, the air and arc gasses 15 being drawn away through a pipe 16 around the negative carbon 2. An image of the positive crater 4 is projected by the optical system 10 on to the slit 8 in the diaphragm 9 and the radiation passing through the slit falls on the photo-electric cell 20 connected to the control grid 21 of a triggering gas discharge'tube 22 to the stabilising grid 26 of which is applied a constant voltage through the resistances 24 and 25. Current from the anode 27 of the discharge tube flows through the coil of an electro-magnetic system 28 controlling a clutch 29 which controls the positive carbon feed mechanism 30 which feeds the carbon at substantially, or slightly faster than, its normal burning rate. The electro-magnetic system 28 comprises an electromagnet 33, Fig. 5, having a winding 32 and a leaf spring armature 34 having an end 43 which can engage the end 42 of the spring 41 coiled partly around a shaft 35, Fig. 6, rotated at constant speed by a synchronous motor and partly around a shaft 38 connected to the feed mechanism 30. The construction may be such that the end 43 of the armature 34 engages the end 42 of the spring 41 when the armature is attracted by the magnet or when it is not attracted by, and is free from, the magnet. In the arrangement shown in Fig. 7 the photoelectric cell 20 and the discharge tube 22 of Fig. 3 are replaced by a tube having a cold cathode 31 having a photo-sensitive electron emissive layer. A resistance 20 A  takes the place of the photoelectric cell 20 of Fig. 3 in order to balance the circuit. In the arrangement shown in Fig. 8 two photo-electric cells 20 B  and 20c are situated behind slits 18 and 18 A  respectively in a diaphragm 19, an image of the end of the carbon 1 being projected on the cell 20 B  and an image of the concentrated arc light being projected on the other cell 20C. In the arrangement shown in Fig. 10 the photo-electric cell 20 of Fig. 3 is replaced by a bi-metallic strip 46. An infra-red filter 47 eliminates a higher percentage of the radiation from the glowing end of the electrode 1 than from the actual arc crater. The filter 47 may be replaced by an absorption filter or reflection filter which allows the short wave radiation to pass through and reflects the long wave radiation. The surface of the bi-metallic strip 46 may have a red colouring or a gold coating which absorbs the short wave radiation and produces a higher reflection of the long wave radiation. A colour selective filter may be introduced in the beam between the arc and its image formed on the radiation sensitive cells embodied in Figs. 3, 7 and 8 in order to intensify the contrast of the projected image. The cathodes of the photo-electric cells used may be made colour selective by using a potassium coating which is particularly sensitive to short wave radiation.