Page:Alan Turing - Proposed Electronic Calculator (1945).pdf/18

 means of a different table, probably a more straightforward but slower one. This should eliminate all errors on the part of the mathematicians, but would leave the possibility of lost cards, etc., when the table is being used a second time. This may perhaps be corrected by running a test job as soon as the cards have been put into the machine.

There are three chief functions to be performed by the checking. It must eliminate the possibility of error, help to diagnose faults, and inspire confidence. We have not yet spoken at all of this last requirement. It would clearly not be satisfactory if the checking system in fact prevented all errors, but nobody had any confidence in the results. The device would come to no better end than Cassandra. In order to inspire confidence the checking must have some visible manifestations. Certainly whenever a check fails to work out the matter must be reported by the machine. There would not be time for all checks which do work out to be reported, but there could be a facility by which this could be laid on temporarily at moments of shaken confidence. Another facility which should have a good effect on morale is that of the artificial error. By some means the behaviour of the machine is disturbed from outside, and one waits for some error to be reported. This could be managed quite easily. One could arrange to introduce an unwanted pulse at any point in the circuit. In fact of course we cannot do very much about checking until the machine is made. We cannot really tell what troubles of this kind are in store for us, although one can feel confident that none of them will be insurmountable. We can only prepare against the difficulties we can foresee and hope that they will represent a large percentage of the whole.

 10. Time-table, Cost, Nature of Work, Etc.

The work to be done in connection with the machine consists of the following parts:

(1) Development and production of delay lines.

(2) Development and production of other forms of storage.

(3) Design of valve-elements.

(4) Final schematic circuit design of LC and CA.

(5) Production of the electronic part, i.e. LC and CA.

(6) Making up of instruction tables.

(7) External organs.

(8) Building, power supply cables, etc.

(1) Delay lines have been developed for R.D.F. purposes to a degree considerably beyond our requirements in many respects. Designs are available to us, and one such is well suited to mass production. An estimate of £20 per delay line would seem quite high enough.

(2) The present report has only considered the forms of storage which are almost immediately available. It must be recognized however that other forms of storage are possible, and have important advantages over the delay line type. We should be wise to occupy time which falls free due to any kind of hold-up by researching into these possibilities. As soon as any really hopeful scheme emerges some more systematic arrangement must be made.

 We/