Page:Alan Turing - Proposed Electronic Calculator (1945).pdf/49

 (1) All the well established forms of storage (excepting the cerebral cortex) are either very expensive and bulky, or else have a very high accessibility time.

(2) The really economical systems consist of layers packed into the form of a solid. They are read by exposing the layer wanted.

(3) The systems which are both economical and fairly fast have the information arranged in two dimensions. This apparently applies even to the cerebral cortex.

(4) Much the most hopeful scheme, for economy combined with speed, seems to be the ‘storage tube’ or ‘iconoscope’ (in J. v. Neumann's terminology).

(5) Some use could probably also be made of magnetic tape and of film for cases where the accessibility time is not very critical.

(iii) Storage tubes. In an iconoscope as used in television a picture of a scene is stored as a charge pattern on a mosaic, and is subsequently read by scanning the pattern with an electron beam. The electron beam brings the charge density back to a standard value and the charge lost by the mosaic registers itself through its capacity to a ‘signal plate’ behind the mosaic. The information stored in this way on an iconoscope, using a 500 line system, corresponds to a quarter of a million digits.

One might possibly use an actual iconoscope as a method of storage, but there are better arrangements. Instead of putting the charge pattern on to the ‘mosaic’ with light we can put it on with an electron beam. The density of the charge pattern left by the beam can be varied by modulating either the voltage of the signal plate or the current in the beam. The advantages of this are:

(a) The charge pattern can be set up more quickly with an electron beam than with light.

(b) Less apparatus is required.

(c) The same beam can be used for reading and recording, so that distortion of the pattern does not matter.

It seems probable that a suitable storage system can be developed without involving any new types of tube, using in fact an ordinary cathode ray tube with tin-foil over the screen to act as a signal plate. It will be necessary to furbish up the charge pattern from time to time, as it will tend to become dissipated. The pattern is said to last for days when there is no electron beam, but if we have a beam scanning one part of the target it will send out secondary electrons which will tend to destroy the remainder of the pattern. If we were always scanning the pattern in a regular manner as in television this would raise no serious problems. As it is we shall have to provide fairly elaborate switching arrangements to be applied when we wish to take off a particular piece of information. It will be necessary to stop the beam from scanning in the refurbishing cycle, switch to the point from which the information required is to be taken, do some scanning there, replace the information removed by the scanning, and return to refurbishing from the point left off. Arrangements must also be made to make sure that refurbishing does not get neglected for too long because of more pressing duties. None of this involves any fundamental difficulty, but no doubt it will take time to develop.