Patent GB-1183587-A (GB)
Title: Electrographic Apparatus
Inventor: BREWSTER ARTHUR EDWARD
IPC: G11B5/49
Filed: 1966-11-11
Abstract: 1,183,587. Charge pattern matrix displays. STANDARD TELEPHONES & CABLES Ltd. 9 Nov., 1967 [11 Nov., 1966], No. 50574/66. Heading G5C. [Also in Division G2] A magnetic or electrostatic charge pattern is formed by a recording head 3 on a recordable medium (which may be e.g. a tape 31) which itself constitutes one wall of a shallow cavity 1 containing a powder for developing the charge pattern to produce a visual image viewable through another wall 2. Means 5 is provided for dispersing the powder particles. The device (as described) may also act as a printer. Viewing cavity (Fig. 11).-The image-developing dust, which may be a magnetic powder, or a dielectric or ferroelectric powder if the device is electrostatic, is agitated in chamber 1 by a blower 5 or by vibrating window 2; or the particles may be fluid-suspended. The back of the chamber comprises a fixed plate (4) (Fig. 1, not shown) or a movable tape 31, which may be non-endless (Fig. 12, not shown), constituting the recording medium, which may be e.g. white or reflecting. The resulting image may be viewed by an image-inverting mirror 36 (Fig. 13, not shown). Print-out arrangements.-A permanent record may be obtained by feeding the signals to a separate conventional non-percussive printing machine, or alternatively directly from tape 31, using the techniques of Specification 1,183,590. Matrix-type recording heads.-As shown in Fig. 2, alphanumeric charge patterns may be formed by a head comprising a ferrite block 8 with slots 9 and coil-carrying apertures 6, (12) (see also Fig. 3, not shown). The slots 9 are narrow and define the recording gaps and provide a " Union Jack "-type monogrammic display. Alternatively (see Fig. 7) the recording head comprises a field of magnetic dots formed by pole-faces 26. Directions are given for making such devices, including the use of epoxy potting compounds and spark erosion techniques. Scanning-type recording heads.-In one embodiment (Figs. 10A, 10B, not shown) a singlegapped head mechanically scans the recording surface alternately in two co-ordinate directions in very small steps, reversing the signal polarity at each step, thereby eliminating all but a fraction of the pattern previously impressed while impressing the next pattern. What eventually remains is the desired pattern of high-definition dots. In other embodiments, non-mechanical scanning is utilized. In Fig. 14B, polepieces (46), (47) energized by an A.C. bias signal through a winding 41 (see also Fig. 14A, not shown) embrace a signal winding 48. The polepieces are made from high-coercivity, rectangular-hysteresis-loop material. The A.C. signal is modulated so as to decay periodically to zero (Fig. 15, not shown). The bias signal path across the gap has a maximum reluctance at "A", a minimum at "B". Thus as the A.C. amplitude diminishes, the location of the threshold, wherein the A.C. signal is only just enough to sweep the hysteresis loop, travels from " A " to " C "; and the final state of a particular domain will depend on the nature of the " message " signal on winding 48 at the time the A.C. signal crossed the threshold. Thus the recording medium 31 or 4 may be scanned between " A " and " C " to impress one line of a display pattern thereon. Relative mechanical movement is necessary in order to impress a succeeding line. A further embodiment utilizes the same principle, but comprises a recording head the length of whose gap varies according to a desired law (e.g. exponentially) (see Figs. 18-21, not shown). A signal comprising a succession of pulses of diminishing amplitude and alternately opposite polarity is applied; a critical magnetic field corresponds to a critical gap length. Thus the recording medium is scanned and a line of pattern impressed. The band 31 is moved past the head to build up a two-dimensional pattern. To achieve precise control of the applied field's magnitude, a plurality of coils having 1, 2, 4, 8 . . . windings are selectively energized in binary code fashion.