Alan Turing did not wait to take up the NPL post before thinking about the practical construction of his universal machine.
In particular he discussed with Don Bayley the problem which dominated its engineering—that of the storage mechanism, or 'tape'.
They discussed every form of discrete storage that they could think of. For instance, they considered magnetic recording.
They had seen a captured German Army 'Magnetophon', the first successful tape recorder,
but rejected the idea essentially because magnetic tape was too much like the tape of the theoretical Universal Turing Machine—it would require so much physical moving to and fro.
Instead, they favoured another solution with which Alan was by now well acquainted—that of the 'acoustic delay line.'
The idea was based on the fact that the time taken for a sound wave to travel along a few feet of pipe was of the order of a thousandth of a second.
The pipe could be regarded as storing the sound wave for that period.
The principle had already been applied in radar, using information stored in the delay line to cancel out all the radar echoes which had not changed since the last scanning.
In that way the radar screen could be made to show only new, or changing, objects.