GB494839A - Improvements in and relating to cathode ray apparatus - Google Patents

Abstract

494,839. Cathode-ray tubes. BRITISH THOMSON - HOUSTON CO., Ltd., and MYNALL, D. J. May 1, 1937, No. 12476. [Class 39 (i)] The invention relates to cathode-ray tubes, with magnetic focusing, particularly for television reception. It is explained in the Specification that when the anodecathode circuit of the tube and the magnetic lens are supplied from a common source, the focus varies with the mains voltage, for the reason that the ideal curve connecting I (the current in the focusing coil) and V (the anode-cathode voltage) has the form I = aV¢ whereas the practical curve is of the form I=bV, b being a constant and a a constant so long as there is no magnetic saturation. According to the invention, the focusing means is so arranged that the curve showing the relation which must hold between I and V and the curve which is obtained under working conditions are so closely aligned over a predetermined range of valves of V that the ray is maintained substantially , in focus over that range. In one arrangement alignment of the curves is obtained by designing the lens to operate in a saturated condition, and in Fig. 5, the lens comprises a magnetic body having an annular air gap C and carrying a winding A. The magnetic circuit is restricted at D and an adjustable magnetic shunt E may be provided to vary the value of flux at which saturation is obtained. In a modification, the restriction is at the side and is shunted by an adjustable plate screwed into the body. These two forms may be used in combination. In a further modification, the required effect is obtained by the use of an auxiliary magnetic field independent of V, and according to Fig. 7, a normal lens J is combined with a permanent magnet K in such a way that the part M carries the difference between the two magnetic fluxes. The field of the permanent magnet may be varied by means of an adjustable element N. The constant field may be obtained by the use of a separate permanent magnet, by means of an auxiliary winding supplied with constant current, or by means of a separate lens supplied with constant current. Adjustments may be dispensed with if the working conditions are not liable to extensive change.