Patent GB-391838-A (GB)
Title: Improvements in and relating to the production of representations of original subjects
IPC: H04N1/024
Filed: 1931-07-28
Abstract: 391,838. 'Valve circuits. DALTON, J. W., 16, Abbott Avenue, Wimbledon, London. July 28, 1931, No. 21555. [Classes 40 (iii) and 40 (v).] A method of producing representations of original subjects, such as printing plates and the like, comprises applying a gradually decreasing negative voltage to the grid of a gaseous discharge tube during regularly spaced predetermined intervals of time, projecting light on to successive small areas of the original in a scanning operation and causing the light reflected or transmitted through each such area to set up a voltage on the grid of the tube proportionate to the tone values of the areas scanned, applying a plate voltage to the tube which is interrupted at the end of each such interval and causing reproducing means, such as a cutting tool, to be actuated by the plate current flowing from the instant at which the grid voltage reaches a predetermined value, permitting the flow of plate current, to the end of each such interval. The discharge tube is preferably of the type known under the registered Trade Mark " Thyratron." The original 1 mounted on a table 2 receives light from a source 39 through holes in a continuously rotating interrupter disc 38, two sets of holes being provided, those 41 serving to transmit light to a light-sensitive cell 43, while those 40 transmit the light rays after reflection from a point on the original to the light-sensitive cell 48. The cells 43, 48 are connected through amplifiers 44, 49 and rectifiers 45, 50, as shown, to the grid circuit of the " Thyratron " valve 60, the anode circuit of which includes the secondary 61 of a primary 62 carrying alternating current from a three-phase synchronous motor 37, and a coil 68 which controls the cutting-tool as described below. The reproduction is obtained on a plate 7 mounted on a table 6, which is reciprocated in two directions at right-angles, on guides 8 by a worm 9 driven from a motor 11, and on guides 13 by a worm 14 rotated at intervals by an electromagneticallyoperated rack 19 and ratchet-wheel 16. The direction of rotation of the motor is reversed to reverse the movement of the table by pins 22 on the table which at the limits of its traverse swing over a contact 24 to engage one or other of the contacts 25, 26 to reverse the current in the motor winding 29. The motor 11, which is on direct current, also serves as a converter to provide alternating current for the transformer 62 and for driving the motor 37, and in order that the reversal of the direction of rotation of the motor 11 may not affect the motor 37, a double-pole phasereversing switch 35 is provided. The motion of the table due to rotation of the screw 14 is obtained at required intervals by the pins 22 which engage switch-over contacts 56, 57, either of which closes the circuit of the electromagnet 21, which thereon attracts its armature and reciprocates the rack 19 to an extent determined by a set screw 58. The oriainal 1 is given a similar movement to the copy 7 by a pantograph device comprising a rod 51 mounted in a ball- and-socket joint 52 on a bracket 53, the height of which can be adjusted. The current variations due to the changes in tone of the original charge the condenser 59 and thereby reduce the bias on the grid of the " Thyratron " valve 60 to which alternating current is also applied by the secondary 73 of the primary 62, and this bias is then further reduced by the current impulses due to the apertures 41 until it attains the critical value at which the " Thyratron " conducts. The time taken for this to occur depends upon the charge given to the condenser by the current impulses due to the apertures 41, so that the valve conducts earlier or later in accordance with the variations of the light reflected to the cell 48, and the time for which it continues is varied correspondingly. The engraving-tool 81 forms a piston in a casing 76, containing fluid which is pumped through a restricted opening 80 by rotary pump 77, and a ball valve 75 is provided under the control of the core 74 of the winding 68. Current flows in this winding for a time which is determined by the tone of the original and thus closes the valve 75 for a varying time, whereby the pressure on the end of the tool 81 is changed proportionally, and thus the depth &c. of cutting is varied in relation to the tone value of the original. The construction may be modified by the use of a mechanical interrupter, when the anode circuit of the " Thyratron " valve is interrupted at intervals, by means of a rotating contact co-operating with contact segments. In the grid circuit is arranged a second rotating contact-maker which engages in succession a number of fixed contact-tappings of a resistance to which a constant potential is applied, so that as the contact maker sweeps over the fixed contacts the negative potential of the grid gradually decreases. Superposed on and partially-neutralizing this grid potential is the potential due to the change in the light received by the light-sensitive cell on account of the tone variation in the original, so that the " Thyratron " tube is caused to light earlier and continue to conduct for a longer period according to the tone of the original. The current flow in the anode circuit is caused to close a valve such as 75 in a fluid-pressure chamber, in which is a spring piston-member operated by a cam. Rotation of the cam forces the piston into the chamber and the pressure inside varies with the time the valve 75 is closed so that the cutting-tool cuts to a greater or less depth according to the tone of the original. The original may be translucent and both the original and copy may be cylindrical and may be mounted on shafts driven in timed relation by a common motor. The invention is applicable to photo-telegraphy, the original being situated at one station while the copy is obtained at another.