An Electronic Computer

The possibilities for a relay computer looked optimistic when Schreyer sud­denly suggested using electronic valves instead. Though they were not then commonly employed for switching be­tween two states, valves could be used in that way and would be far faster than relays. "At first I thought it was one of his student jokes – he was always full of fun and given to fooling around", Zuse has recalled.

About 2000 valves would be needed. Asking for them, and getting them, were two different things in a Germany then at war. Private enterprise stood no chance4 so they talked to the German Army Command. Whilst the initial reaction was favourable, the idea foundered when they said it would take about two years to build. "And just how long do you think it'll take us to win the war?" they were asked.

So little help came, but by the end of the war Schreyer had built an ex­perimental computer with just 100 to 150 valves, and gained his doctorate on the way for his work on valve switching circuits. Like the other computers, this too was a casualty of the war. After the war the development of electronic equipment was banned in Germany and so Schreyer emigrated to Brazil. It was there that he died in 1985.

Whilst Schreyer worked part–time on the electronic machine Zuse completed the electromagnetic relay computer, the Z3, encouraged by the Experimental Aircraft Institute. The Z2 had con­vinced the Institute of the usefulness of Zuse's ideas and so it financed the Z3, though Zuse still had to work alone and at home. And he had to escape a recall to active duty for service5 on the Eastern Front.

The Z3 was the first general–purpose digital computer in the world. It was completed in 1943. It employed binary numbers, floating-point arithmetic and a 22-bit wordlength, and it has been estimated that it used around 2000 re­lays (and eight uniselector switches) and cost the equivalent of between $6000 and $7000. "The most important thing", says Zuse, "seemed to be to keep the frequency absolutely even, so that one cycle equaled one addition". This he achieved using a rotating disc or roller, each revolution defining one operation. As the disc's speed could be varied, so too could the operating speed of the computer. Sparking at the relay con­tacts was eliminated by making or breaking them before any current flowed, so increasing reliability. Post­war Zuse machines are said to have been "legendary" for their reliability."

Although the Z3 was completed (with the help of friends) it served mainly as an experimental machine and it never went into routine use probably because of the limited capacity of its memory. There are no doubts, however, that it was fully functional, because there are several witnesses to its operation. Though the original Z3 was blitzed out of existence6 a reconstruction was made years later, based on the surviving patents, and is now in the Deutsches Museum in Munich.

The Survivor

Somehow Zuse found time to build other computers as well. The S1 was a non-programmable machine using hard-wired programs. It served in the design of the Henschel flying bomb HS-293, a pilotless aircraft guided by radio from a bomber. It replaced a dozen calculators. An improved design, the S2, was too late for routine service and is the one that Zuse thinks might have been captured by the Russian army. But the big one was the Z4: a full-sized general-purpose computer, the only one to survive the war.

Construction of the Z4 began in 1943, even before the Z3 was finished. For this large machine Zuse returned to his successful mechanical memory design. Whilst this now seems a retrograde step it was the only way he could achieve a large memory (1024 32-bit words) in a reasonable volume.

Work on the computer began in Ber­lin but Allied bombing posed an everpresent threat7. "My workshop was damaged several times, and three times during the war we had to move the Z4 around Berlin." As allied bombing increased in 1945, the authorities de­cided to move Zuse and his new compu­ter out of the capital to Göttingen, 160 miles to the west. There construction was completed and on April 28, 1945, demonstration programs were run for the authorities. "This was the moment for which I had waited for 10 years–when my work finally brought the suc­cess I desired." The irony for Zuse was that the machine was immediately dis­mantled, because the American army was by then just a few miles away.

The odyssey continued as they were ordered to underground works in the Harz mountains where the V1 and V2 weapons were being built. Zuse has described the conditions there as terrible. "We refused to leave the machine there." With great difficulty it was moved to an alpine village just north of the Austrian border where it was set up in a barn. There it stayed until 1949 when it was rescued, rebuilt and estab­lished in the Technical University in Zürich in 1950. For a time it was the only functional digital computer on the continent.

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