Patent GB-686527-A (GB)
Title: Improvements in or relating to focussing coils for electron beams
Inventor: PEARCE RALPH REGINALD
IPC: H01J29/66
Filed: 1948-08-10
Abstract: 686,527. Electron lenses. ELECTRIC & MUSICAL INDUSTRIES, Ltd. July 30, 1949 [Aug. 10, 1948; Nov. 3, 1948], Nos. 21026/48 and 28508/48. Class 39(i) [Also in Group XXXV] A focusing coil for a receiving cathode-ray tube comprises a flat conductor wound as a planar spiral or a plurality of short interconnected coil units each formed as a planar spiral arranged side by side, and the current supply connections are such as to produce an axial flow of current in opposite directions so that the resultant axial current flow is negligible. In one form, Fig. 2, the coil 7 is a strip of aluminium foil 0.001 inches thick and 2 inches wide and adjacent turns are insulated by paper 10 or by anodising the aluminium. Metallised paper may be used instead of foil. The inner end of the coil is wound on and connected to a copper former 9 both ends of which are connected to one terminal of current source 11, the other terminal of which is connected to the centre point of the outer end of the coil. The axial length of the coil is at least one half its minimum diameter. In another form, Fig. 3 (not shown), using an odd plurality of coil units, each coil unit may be of wire or narrow tape. Adjacent turns may be secured together by an adhesive or alternatively a ring of insulating material may have coils secured thereto on one or both sides. In another form, Figs. 5, 6, a methyl methacrylate ring is provided with spiral grooves 20, 21, one groove being clockwise and the other anticlockwise, which are then filled with copper by electro-deposition. The inner turns of the spiral grooves merge into annular grooves 24, 25 which when filled with metal provide contact rings 26, 27, and the outer ends of the spirals merge into contact areas 28, 29 connected to a contact eyelet. The inner ends of adjacent spirals of adjacent insulating rings are connected by metal rings 32, the other turns being insulated by paper rings 33 or by insulating layers applied to the coil units. The insulating rings are piled up and clamped together, the connections between the coils being as shown in Fig. 6, A indicating the clockwise wound coils and C the anticlockwise wound coils. Since the two centre coils C do not carry current and coils A are wound in the same direction the device effectively comprises an odd plurality of coils as in Fig. 3 (not shown). In another method of manufacture, a ring of phenolic resin, mica, or paper, is metallised on both sides by securing metal foil to it or by electrodeposition. The spiral coil pattern together with contact rings or areas is then painted, printed, or photographically reproduced on to the metallised surface and unrequired metal areas are then removed as described in Specification 602,492, [Group XXXV]. Insulating layers may be applied to the coil units. In a modification of this method, the spiral coils may be formed by spraying metal on to an insulating surface through a stencil. In another modification a tool may be applied to an insulator carrying a layer of metal foil to cut away the foil and so form the required spiral. The coil arrangements of the invention are such that an axial flow of current is reduced to a minimum and thus aberrations in focusing, allegedly due to axial components of current in the coils, are also reduced.