The Z2, Z3, and Z4
Zuse completed his work entirely independently of other leading computer scientists and mathematicians of his day. Between 1936 and 1945, he was in near-total intellectual isolation. In 1939, Zuse was called to military service, where he was given the resources to ultimately build the Z2. In September 1940 Zuse presented the Z2, covering several rooms in the parental flat, to experts of the Deutsche Versuchsanstalt für Luftfahrt (DVL; i.e. German Research Institute for Aviation). The Z2 was a revised version of the Z1 using telephone relays.
The DVL granted research subsidies so that in 1941 Zuse started a company, Zuse Apparatebau (Zuse Apparatus Construction), to manufacture his machines, renting a workshop on the opposite side in Methfesselstraße 7 and stretching through the block to Belle-Alliance Straße 29 (renamed and renumbered as Mehringdamm 84 since 1947).
Improving on the basic Z2 machine, he built the Z3 in 1941. On 12 May 1941 Zuse presented Z3, built in his workshop, to the public. Z3 was a binary 22-bit floating point calculator featuring programmability with loops but without conditional jumps, with memory and a calculation unit based on telephone relays. The telephone relays used in his machines were largely collected from discarded stock. Despite the absence of conditional jumps, the Z3 was a Turing complete computer (ignoring the fact that no physical computer can be truly Turing complete because of limited storage size). However, Turing-completeness was never considered by Zuse (who had practical applications in mind) and only demonstrated in 1998 (see History of computing hardware).
The Z3, the first fully operational electromechanical computer, was partially financed by German government-supported DVL, which wanted their extensive calculations automated. A request by his co-worker Helmut Schreyer—who had helped Zuse build the Z3 prototype in 1938—for government funding for an electronic successor to the Z3 was denied as "strategically unimportant".
In 1937, Schreyer had advised Zuse to use vacuum tubes as switching elements; Zuse at this time considered it a crazy idea ("Schnapsidee" in his own words). Zuse's workshop on Methfesselstraße 7 (with the Z3) was destroyed in an Allied Air raid in late 1943 and the parental flat with Z1 and Z2 on 30 January the following year, whereas the successor Z4, which Zuse had begun constructing in 1942 in a new premise in the Industriehof on Oranienstraße 6, remained intact. After devastating destructions in the Luisenstadt, the area around Oranienstraße, including the neighbouring houses on 3 February 1945, the partially finished, relay-based Z4 was packed and evacuated from Berlin on 14 February, only arriving in Göttingen two weeks later.
Work on the Z4 could not continue in the extreme privation of post-war Germany, and it was not until 1949 that he was able to resume work on it. He showed it to the mathematician Eduard Stiefel of the Swiss Federal Institute of Technology Zurich (Eidgenössische Technische Hochschule (ETH) Zürich) who ordered one in 1950. On 8 November 1949, Zuse KG was founded. The Z4 was delivered to ETH Zurich on 12 July 1950, and proved very reliable.
While Zuse never became a member of the Nazi Party, he is not known to have expressed any doubts or qualms about working for the Nazi war effort. Much later, he suggested that in modern times, the best scientists and engineers usually have to choose between either doing their work for more or less questionable business and military interests in a Faustian bargain, or not pursuing their line of work at all.
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