Alan Turing: The Enigma (86 page)

^*
In November 1946 he had also tried to retrieve a spare Delilah power pack from the Cypher Policy Board: ‘Do you think it could be spared and sent down to me here? It is an old friend whose tricks I am used to.’ But this was almost certainly unsuccessful.

^*
As further illustration, it is noteworthy that irrespective of Alan’s reports, Womersley continued to assume that the Americans held all the trumps. In April 1947 he daringly suggested that Darwin, currently attending meetings of the United Nations in New York, should call at Princeton to negotiate for the latest news in edvac engineering. This proposal was ill-timed; in May the NPL was declared by the American armed services to be ‘unsuitable’ for the receipt of such (commercially valuable) information. The ruling was relaxed later in 1947: information could be passed to Britain provided it was used
only
for military purposes.

^*
Giving the whole £400 was generous, but illogical. It was put in trust for his nieces.

^*
He also received reduced pay at the rate of £630 per annum for the sabbatical period. Darwin had offered full pay, but Alan told him he would prefer half-pay, saying that on full pay he would feel that ‘I ought not to play tennis in the morning, when I want to.’

^*
Higher than that of ‘lecturer’, but not as high as a true ‘Prof.

^*
The actual words he used were ‘Universal Practical Computing Machine’,

^†
A false prophecy.

^*
He meant what has here throughout been called ‘cryptanalysis’, as the following passage shows.

^*
i.e
. ‘infinity’. This was mathematical shorthand for the fact that a Babbage-like machine could in principle be fed with an
unlimited
quantity of data and instructions from an external source – the price being, of course, an unlimited time delay.

^*
A remark very characteristic of his post-war life – but there is no evidence as to how this particular contact had come about.

^†
As The Times put it on 9 August, the public would ‘put the true high value on the near misses of the British men and women.’

^*
As it happened, Alan and his mother had seen Noyce as a boy, passing him when walking in the Welsh hills in 1927.

7

The Greenwood Tree

Unseen buds, infinite, hidden well,

Under the snow and ice, under the darkness, in every square or cubic inch,

Germinal, exquisite, in delicate lace, microscopic, unborn,

Like babes in wombs, latent, folded, compact, sleeping;

Billions of billions, and trillions of trillions of them waiting,

(On earth and in the sea – the universe – the stars there in the heavens,)

Urging slowly, surely forward, forming endless,

And waiting ever more, forever more behind.

What Alan Turing did not know was that
a number of changes had been made at Manchester University since his appointment in May. He had been created ‘Deputy Director’ of the ‘Royal Society Computing Laboratory’ when Newman was supposed to be directing it, and the Royal Society funding it. But by October it had become clear that F.C. Williams had need neither of a ‘Director’ nor of the Royal Society.

In the development of electronic hardware, the important factor had been that Williams’s ingenuity was backed by a cosy relationship with TRE, which allowed him to draw upon their supplies, and to have two assistants seconded from the government establishment. One of these was a young engineer with a Cambridge mathematics degree, T. Kilburn. The second, after a short interval, came to be G.C. Tootill, another young TRE man from the same wartime Cambridge year.

As for the development of a logical design, the first step had been taken by Newman. He had explained the principle of storing numbers and instructions, which according to Williams
¹
‘took all of half an hour’, favouring the von Neumann type of design.
^*
In late 1947 the plans had rapidly evolved in the hands of Williams and his two assistants. They were not detained by the prospect of ‘formidable mathematical difficulties’, but
had pressed on, as Williams put it,
^*
‘without stopping to think too much’. The result was the tiny computer of whose existence Alan had learned in the summer, whose store consisted of just one cathode ray tube.

The advantage of the cathode ray tube over the delay line was that, in both senses, it eliminated delay. It was essentially an ordinary piece of equipment, not requiring precision engineering as did the mercury delay line, and could be taken ‘off the shelf’. In practice this virtue was tempered by the fact that most tubes contained too many impurities in the screen to be used, but its ‘home-made’ quality was still of value in getting the project off the ground. In operation it was not particularly fast – indeed it would take ten microseconds to read a digit as compared with the one microsecond intended for the ACE – but this was compensated by the fact that the information stored on the tube was directly available, without the long period of waiting for a pulse to emerge from a delay line. Continuing his ‘papyrus scroll’ analogy, Alan compared it
³
to ‘a number of sheets of paper exposed to the light on a table, so that any particular word or symbol becomes visible as soon as the eye focusses on it.’

They had been able to store 2048 spots on the tube by the principle of regenerating them periodically, but in the end had settled on using just 1024, arranged in thirty-two ‘lines’ each of thirty-two spots. Each line would represent either an instruction or a number. A second cathode ray tube served as the logical control, storing the instruction currently being executed, and the address of that instruction. A third acted as the accumulator, the shunting station for the arithmetical operations. It was a ‘one-address’ system, so that each act of shunting in, or of shunting out, constituted a full instruction – an arrangement entirely different to that envisaged for the ACE. Arithmetic was, however, reduced to the barest minimum for the sake of demonstrating that it was possible at all – the operations of copying and subtraction, together with a simple form of conditional branching. It amounted to far less than Huskey’s ‘Test Assembly’ would have done, had that abortive NPL effort been completed. Physically, the Manchester computer was embodied in a straggly jumble of racks and valves and wires,
^†
with three screens glowing in the gloom of a room with dirty brown tiles which Williams was fond of describing as ‘late lavatorial’ in style.

It was, in fact, the most
obvious feature of the cathode ray tube storage But this was enough. As Williams described
⁴
the day of triumph:

When first built, a program was laboriously inserted and the start switch pressed. Immediately the spots on the display tube entered a mad dance.

In early trials it was a dance of death leading to no useful result, and what was even worse, without yielding any clue as to what was wrong. But one day it stopped and there, shining brightly in the expected place, was the expected answer.

This happened on 21 June 1948, and the world’s first working program on an electronic stored-program computer, to find the highest factor of an integer by crude brute force trial, had been written by Kilburn.

Nothing was ever the same again. We knew that only time and effort were needed to make a machine of meaningful size. We doubled our effort immediately by taking on a second technician.

It was in these circumstances that Kilburn mentioned to Tootill a few days later that ‘there’s a chap called Turing coming here, he’s written a program.’ Williams knew about Alan because of his dealings with the NPL. Kilburn vaguely knew of him. Tootill, who had not heard of him at all, worked on the program. He was astonished (and naturally, smugly pleased) to discover it not only to be inefficient but to contain an error.

At Manchester they had a machine which actually worked, and this simple fact counted for more than did ingenious or impressive plans. It meant that while Alan had been away on his holidays, political considerations had transformed the Manchester set-up. Already in July, Sir Henry Tizard, then Chief Scientific Adviser to the Ministry of Defence, had seen the machine and considered it
⁵

of national importance that the development should go on as speedily as possible, so as to maintain the lead which this country has thus acquired in the field of big computing machines, in spite of the large amount of effort and material that have been put into similar projects in America. He promised full support both in supply of material and in obtaining necessary priorities.

To the engineers it was a gratifying verdict, but it was one which had no connection whatever with the ‘fundamental research in mathematics’ that was Newman’s object, and the purpose of the Royal Society grant.

It was not surprising that Tizard should take this view. In 1948 (although he changed his mind in 1949, saying that Britain should admit it was no longer a Great Power), he supported the policy of building a British atomic About £100,000 was thus spent
by the government, whose rapid, almost panicky move made a strong contrast with the stately progress of Planned Science at the NPL. It had more to do with events in Berlin and Prague than with the intentions of the Royal Society. (It was in that same month of October 1948 that the demolition of air raid shelters was suddenly stopped.) It certainly had nothing to do with Alan, the pawn in the Great Game. For that matter the
carte blanche
contract made no reference to Newman or Blackett. Newman’s motives had been entirely those of a pure mathematician, one who wistfully thought of what the talent at Bletchley could have achieved had it had been applied to his subject. He had originally wanted to buy a machine and get on with the mathematics, and by this time had realised that it could not be so; the development of the hardware was going to be a dominating feature, and his interest had accordingly waned. He therefore did not object to the project being taken away. Blackett, however, was distinctly annoyed, perhaps the more so as he opposed the atomic weapon development.

But even apart from the politics of the machine, Alan had come too late to direct its development. Already the important decision had been taken to adopt, for use as a large, slow, backing store, a rotating magnetic drum such as A. Booth of Birkbeck College, London, had developed for use with a relay calculator. With digits stored on tracks around the drum to be read off by a head, this was equivalent to providing a large number of slow, cheap, delay lines for the storage of data and instructions not immediately in use. Another innovation in the design, a modification originally suggested by Newman, was that of the ‘B-tube’. (It was so called because the arithmetic and control tubes were naturally ‘A’ and ‘C’ tubes respectively.) This additional cathode ray tube had the property of modifying the instructions held in the control; in particular it could be used when working along a sequence of numbers, in such a way that the idea of the ‘next’ number did not have to be rendered into laborious programming.
^*
As such it was contrary to the general policy that Alan had pursued on the ACE design, that of using instructions rather than hardware as far as possible.

But more generally, the design and development had all been decided by others. They called it the ‘baby machine’ – but it was someone else’s baby. Williams had turned the tables, for while Darwin had hoped for him to build to Alan Turing’s instructions, now Alan had the task of making Williams’ machine work. With the best will in the world, there was room for conflict; the more so as the engineers had no intention of being ‘directed’ by anyone. The line between ‘mathematicians’ and ‘engineers’ was demarcated very clearly, and if not quite an Iron Curtain, it was a barrier as awkward as the MacMahon Act. This would never be Alan Turing’s machine, as the ACE would have been, and correspondingly, he withdrew as much as possible from any administrative responsibility for it. But he could foster it, and there was the prospect of using it. His position also attracted the salary of some £1200 per annum (increased to £1400 in June 1949), and very considerable freedom.