Patent GB-237263-A (GB)
Abstract: 237,263. Marconi's Wireless Telegraph Co., Ltd., (Assignees of Ranger, R. H.). July 16, 1924, [Convention date]. Thermionic valve circuits.-In a copying-telegraph system an arrangement of thermionic valves is employed to ensure that high lights in the picture are transmitted as uniform short impulses or dots separated by varying spacing periods, as described in Specification 230,075 while light values below a certain intensity are represented by impulses varying in length according to intensity of shadow separated by uniform spacing periods. At a certain intermediate tint the impulses and spacing periods are equal in duration. In the arrangement shown in Fig. 1, condensers 16, 17 associated with valves 18. 19 alternately discharge through coils 32, 33 thus connecting + and - terminals of battery 34 to line. The rate of charge of the condensers 16. 17 is controlled by valves 14, 15 the grid circuits of which are controlled by valves 6, 7 respectively. Valve 7 is controlled by valve 6 which in turn is controlled by a photo-electric cell on which the picture is projected in the usual way. Biasing batteries 8, 9, 13, 20. 21 and resistances 10, 11. 12, 27, 28 in the plate circuits of the several valves are arranged as shown. Normally no current flows through the valves 14, 15. The condenser 17, when the armature 22 is as shown, charges through coil 33 and resistance 29, condenser 16 being short-circuited by the armature 22. The bias of battery 21 on the grid of valve, 19 is overcome, plate current flows through the coil 26 and armature 22 reverses. Condenser 17 discharges through coil 33, contact 24, and armature 22 and the armature 29 sends current to line. Condenser 16 is now free to charge up and a similar sequence of operations takes place. If valves 14, 15 are not passing any plate current a series of equal marking impulses equally spaced is thus sent to line. High lights on the picture, however, cause a high negative potential to be impressed on the grid of valve 6, the plate thereof rises in potential, and the higher potential overcomes the bias of battery 9 causing plate current to flow in valve 14. This flow delays the charging up of condenser 16, and prolongs the spacing intervals during which armatures 22, 29 are over to the right. At the same time the increased potential on the plate of valve 6 overcomes the bias of battery 8 on the grid of valve 7 and causes plate current to flow in the valve, thus lowering the potential on the grid of valve 15, so that plate current is stopped and condenser 17 can charge up in the usual time. On the other hand if shadows occur, the grid of valve 6 is positive, the grid of valve 7 becomes negative and plate current flows in valve 15 prolonging the marking intervals. The armature 29 may be omitted and armature 22 used to control the transmitter. A slight overlap in the action of the grids of valves 14 and 15 is arranged so that plate current is not completely cut off in one until it has commenced to flow in the other. In order to reduce the use of mechanical relays, armature 22, and contacts 23, 24 are eliminated in the arrangement shown in Fig. 2. Valves 36, 37 correspond to valves 14, 15 in Fig. 1 and condensers 42, 43 to condensers 16, 17. Condensers 42, 43 charge from source B through resistances 38, 39. When condenser 42 changes up, bias battery 44 is overcome and plate current of valve 46 flows through resistance 48. When condenser 42 has taken its maximum charge, it is arranged that condenser 43 has already begun to charge and has overcome the effect of battery 45 allowing valve 47 to begin to pass plate current and causing a sudden drop in the potential of the plate, and consequently of the plate of condenser 51 connected thereto. Condenser 42 therefore discharges through coil 52 and condenser 51. The potential of the grid of valve 46 drops, and the valve ceases to take plate current: condenser 42 immediately begins to charge again and valve 4G to take plate current; and condenser 43 discharges through condenser 50 and coil 53. The coils 52, 53 mav be proportioned to build up the charges on condensers 42, 43 to a high value but the charges are kept from exceeding a definite limit by the tendency of valves 46, 47 to pass grid current. To assist this, valves passing large grid current are used. The cycle of operations always starts from the same point and the marking periods will be controlled in duration by the intensity of the light regardless of the conditions on the spacing side and vice versa.