GB547998A - Magnetic focussing arrangements for cathode ray tubes - Google Patents

Abstract

547,998. Image dissectors. STANDARD TELEPHONES & CABLES, Ltd. Sept. 19, 1941, No. 12062. Convention date, Sept. 28, 1940. [Classes 39 (i) and 40 (iii)] To reduce "spiral" distortion in image dissector tubes wherein the electron image at the scanning plane is of greater area than that produced at the photo-cathode, the electro-magnetic focusing field is varied along the tube so that the magnetic lines of force follow the " expanding" electrostatic field. The electro-magnet extends beyond the ends of the tube 10 and in one form comprises two superposed windings each divided into corresponding sections 41, 43 ... 71, 73 and 40, 42 ... 70, 72 ; the number of turns in the sections are graded so that the numbers increase in the odd numbers section 41 having least, while the grading is reversed in the even numbered, 40 having most number of turns. As a consequence by selecting the current to be sent through the sections the tapering of the field can be controlled. The current is supplied from a rectifier 80 through three variable resistances 87, 88, 89, that from 87 passes to coils 44 to 72, from 88 through coils 45 to 73 and from 89 only to coils 40, 42 to give end correction ; similar end correction may be made at the other end of the tube. The tapering field may also be obtained by using (i) a single coil with number of turns varying uniformly along the tube; (ii) coils spaced much closer at one end than at the other ; (iii) coils uniformly spaced but carrying differing currents each coil being shunted by a resistance. The image produced at the photo-cathode 21 is deflected by coils arranged inside the focusing coils over an orifice 27 in an electron multiplier 22 ; only a portion of the surface of the cathode 21 is used for emission.