Patent GB-890780-A (GB)
Title: Improvements in or relating to information storage systems
Inventor: AYERS SHEILA
IPC: G11C11/22
Filed: 1957-05-16
Abstract: 890,780. Circuits employing bi-stable dielectric elements. POSTMASTER-GENERAL. Aug 11, 1958 [May 16, 1957], No. 15645/57. Class 40 (9). [Also in Groups XIX and XXXVI] Binary information is stored in a ferroelectric crystal 1, Fig. 1, formed of material such as barium titanate, mixed titanates of barium and strontium or guanidine aluminium sulphate hexahydrate, by applying a writing pulse between grouped electrodes 2, 4 and 3, 5 so as to produce a lateral polarization parallel to the major crystal surfaces, and a storage condition is cancelled by applying a restoring pulse between grouped electrodes 2, 3 and 4, 5, whereby a polarization normal to the major surfaces is produced, the two kinds of polarization respectively switching the optic axis of the crystal through a 90 degrees angle so that the crystal becomes transparent to polarized light only when laterally polarized by a writing pulse. The polarized light transmitted by the crystal may be visible or infra red and is detected by a photo-electric device which provides a continuous read-out indication of the information stored. An array of storage cells may utilize a common crystal block as in Fig. 3, in which four cells are provided by the use of eight separate electrodes 15, 16 on one crystal surface and two common electrodes 13, 14 on the other surface. A larger array is shown in Fig. 4 in which the separate electrodes 17 on one surface are grouped by connecting links mounted on insulating bars 19. In both constructions the electrodes and links are formed by evaporating material such as silver on to the crystal surfaces. A matrix of storage cells is provided in the embodiment shown in Fig. 5, parallel electrodes 19, 20 being provided on respective major surfaces of a crystal in column and row relationship. Switching of a selected cell is effected by applying voltages of V/2 and -V/2 to a total of four electrodes, two from each crystal surface, V being the voltage necessary to effect a switching operation. The number of electrodes necessary may be reduced by using a stack of crystals through which the polarized light passes successively. This principle is illustrated in Fig. 7 in which four storage cells are provided by electrodes 26-33 and 35-42 of respective crystals 25, 34. When four electrodes of each crystal are appropriately energized by a writing pulse, a light path is established through both crystals over one quarter of the surface area. The application of this method to an 8 X 8 array of storage cells using a stack of six crystals is outlined with reference to Fig. 8 (not shown). A resistive circuit for selectively applying the two kinds of polarization to the electrodes E1 of a single cell is shown in Fig. 9, lateral and normal polarizations being respectively obtained by pulses between terminals T1 and T3 or T2 and T3. Suitable optical systems for use with a multiple storagecell crystal are also described, Figs. 10-12, each system utilizing a polarizer 61 and analyzer 62. In Fig. 10, a lamp 48 illuminates the whole of a crystal 49, and a lens 50 forms an image on an array of photo-electric devices 51, such as photo-transistors, so that each device responds to light passing through a particular storage cell. Alternatively, a projection type cathode-ray tube 52, Fig. 11, with a fluorescent screen is used as a scanning source of light in association with lenses 53, 55 and a single photo-electric device 56. A further arrangement, Fig. 12 (not shown), utilizes a stack of crystals including an auxiliary crystal particular regions of which may selectively be made light conductive, and a single light source and photo-electric device.