Konrad Zuse developed the world's first mechanical and electromechanical computers called the Z1, Z2, and Z3. Written by Konrad's eldest son, Horst, this article features many hitherto unpublished photographs from Horst's private collection. In this installment we discuss the development of the Z4 and the Zuse Apparatebau.

The Z4 Computer and the Zuse Apparatebau in Berlin (1940-1945)
In 1940, Konrad Zuse formulated a vision, which was to use computers to release the spirits of humans from the stupid task of calculations. In order to build his computers, Konrad Zuse founded a company called the Zuse Apparatebau in Berlin on April 1, 1940.

The Zuse Apparatebau was a small company located in the Methfesselstraße 7 and 10 in Berlin-Kreuzberg. By 1941, Konrad Zuse was sure that the general problems of building powerful computers were solved. He planned the Z4 to be a prototype of computers for engineering bureaus and scientific institutes. Based on his experiences with the Z1 to Z3 machines, and knowing the problems he wanted to solve for the engineers, he realized that the Z4 needed much more memory than in the previous machines.

For this reason, he compared the advantages and disadvantages of a memory built using relays to a memory constructed from thin metal sheets (like the Z1 and Z2). His conclusion was that constructing the memory from metal sheets was much less expensive than building a relay memory. It was clear to him that a memory of one thousand 32-bit words consisting of relays would be too big, because he would need more than 1000 x 32 = 32,000 relays. His patented mechanical memory (1936) worked very reliably, and for 1000 words he would not need more than one cubic meter of space. Konrad Zuse also estimated the costs of one 32-bit word of his mechanical memory as being 5 RM (Reichmarks), which equated to approximately $2.50 US in 1942.

By 1945 the Zuse Apparatebau had about 20 employees. Unfortunately, at this time the building and the company were completely destroyed by air raids,. To this day, the plot of land in the Methfesselstraße 7 is a bombed site. Figure 7-1 shows the Methfesselstraße 7 and 10, where Konrad Zuse built the Z3 and started the construction of the Z4. Figure 7-2 shows Konrad Zuse at the entrance of the Methfesselstraße 7

Figure 7-1. Methfesselstraße in Berlin (1999). The Z3 and Z4 were constructed
in a house on this plot of land (just to the right of the red car).

Figure 7-2. Konrad Zuse (1989) at the entrance of the Methfesselstraße 7.

The Z4 Computer
The goal of the Z4, which was developed between 1942 and 1945, was to build the prototype for a machine that was intended to be produced in the thousands. Unfortunately, the war destroyed my father's hope, which was that his machines should support the work of engineers of the time.

Figure 7-3. The Z4's input and output devices, as drawn in 1942 by Konrad Zuse.

Figure 7-4. Konrad Zuse at the building of the Oranienstraße 6 in
Berlin-Kreuzberg (1989), where the Z4 was almost finished in 1944/45.

It took more than four years to build the Z4, which ended up being much smaller than was originally planned. In the case of the Z4, Konrad Zuse wanted to implement a sub-program principle. For this reason he planned six punch tape readers and two punch tape writers. However, lack of materials, the almost daily air raids (Figured 7-5 and 7-6), and the increasing difficulty of living in Berlin (which worsened daily) made it impossible to finish the Z4 completely.

Figure 7-5. The area around the Brandenburger Tor (Brandenburg Gate) in 1945.

Figure 7-6. The Brandenburger Tor in 1947.

The Z4 after the War
Most people today cannot imagine how bad the situation in Germany in 1945 was. Berlin was attacked every day and night by more than 800 bombers. Konrad Zuse eventually fled from Berlin with his pregnant wife Gisela and the Z4 in March 1945. The Z4 itself was transported on a military truck. This was made possible by some confusion between the Z4 computer and the V2 rocket. At that time, Konrad Zuse denoted his machines as V1, V2, V3 and V4, where the 'V' stood for "Versuchsmodell", meaning "trial models". Thus, the military mistakenly believed that Konrad Zuse had parts of the V4 rocket, a successor of the V2 rocket (of course my father and the Z4 (V4) computer had nothing whatsoever to do with these rockets).

However, due to this confusion, may father gained access to a military truck, which was used to transport the Z4 to Göttingen (300 km away from Berlin). For this trip, which took one week, they could only move during the night because there were low-level aircraft attacks in the daytime. During the day the party found makeshift shelter with farmers. In Göttingen Konrad Zuse presented the Z4 to the Professors Prandtl and Küssner of the Aerodynamsche Versuchsanstalt (areodynamics research institute).

It was originally intended that the Z4 should be delivered to the underground factories in Nordhausen, where the rocket V2 was in serial production. However, Konrad Zuse was deeply shocked by what he discovered there. Although he was by now completely accustomed to the bombings in Berlin, he now saw the inhuman atrocities of the Third Reich with his own eyes. Twenty thousand concentration camp prisoners worked under unimaginable conditions in kilometer-long tunnels. After seeing this, Konrad Zuse declared: Anywhere, but not here. Fortunately it was possible to take the truck southeast to Hof, then via the burning Munich to Kloster Ettal, and finally to Haus Ingeborg in Oberjoch (Bavaria) in the alps, where he arrived in April 1945.

In the Haus Ingeborg, which is about 10 km from Hindelang (Allgäu, Bavaria) Konrad Zuse met Wernher von Braun (see note) with part of his group (about 100 persons) and General Dornberger for the first and last time in his life.

Note: Wernher von Braun was the head of the team in charge of designing and constructing the V2 rocket in Peenemünde in the north east of Germany (General Dornberger was the commander in Peenemünde). After the war, Wernher von Braun moved to US, where he could proceed with his work on rockets. Wernher von Braun was later involved in satellite projects and the Apollo moon rocket.

My father was able to have a brief discussion with Wernher von Braun, and he later said that von Braun had not forseen the role computers were to play in the future. After a few days Konrad Zuse left the group and proceeded to Hinterstein, which was about 20 km away from Oberjoch, because he did not want to be involved with the V2 rocket in any way. In Hinterstein my fasther hid parts of the Z4 in a barn of the Hotel Steinadler.

Figure 7-7. The Hotel Steinadler in Hinterstein (Allgäu).

Figure 7-8. The barn where the Z4 was hidden from 1945-1946.

The situation around Hinterstein was complicated. For example, there were more than 1200 refugees, 200 soldiers, and so forth. Konrad Zuse and his wife found a room in the House Tannheimer. From 1945 till 1947 it was almost impossible to restore the damaged Z4, which was, at this time, the only computer in Germany (and probably in Europe). In fact, it is interesting to note that the Z4 was probably the only computer in Europe until 1951!

Konrad Zuse desperately wanted to resume work on the Z4, but his first problem was to survive the years after the war. In order to get some food, he made woodcuts and sold them to the farmers and the American troops. Figure 7-10 shows a woodcut by Konrad Zuse, which depicts Hinterstein. The Z4 computer was hidden close to the church.

Figure 7-9. The small village of Hinterstein (1985).

Figure 7-10. A woodcut of Hinterstein by Konrad Zuse (1946).

Figure 7-11. The House Tannheimer in Hinterstein, where Konrad Zuse and his family
found room on the first floor (the four windows at the front edge of the house).

Figure 7-12. A woodcut of the House Tannheimer by Konrad Zuse (1945).

Figure 7-13 shows a three color woodcut (60cm x 80cm) of the waterfall Zipfelsfälle in Hinterstein by Konrad Zuse. He used three separate wood plates, one for yellow, one for blue, and one for red (the green color was created from the colors yellow and blue). Each wood plate was pressed onto the same piece of paper in turn to create the final image. Figure 7-14 shows a woodcut by Konrad Zuse, which shows the hut Roßhütte and the Erzberg mountain close to Hinterstein in the winter of 1945/46. And Figure 7-15 shows a woodcut of the church in Hinterstein in 1946.

Figure 7-13. A three color woodcut of a waterfall in Hinterstein.

Figure 7-14. A woodcut by Konrad Zuse in the winter of 1945/46.

Figure 7-15. A woodcut of the church in Hinterstein in 1946.

The Z4 after the War
During the period from 1945 to1947 it was almost impossible to fully restore the damaged Z4. However Konrad Zuse used tin cans he obtained from the allies to repair parts of the Z4's mechanical memory.

Unfortunately there was nowhere in Hinterstein that was suitable for Konrad Zuse to set up the Z4. Thus, in 1947, Konrad Zuse, his wife Gisela, and their son (myself – Horst) moved to Hopferau (Bavaria), which is about 30 km north of Hinterstein. By 1948, the Z4 was reassembled in a former flour store in Hopferau. Some calculations could be performed, but the electrical power was only available for a few hours each day, and it was generally not sufficient to accomodate the Z4's power consumption.

In 1948 Eduard Stiefel from the ETH-Zürich wanted to set up a new Institute for Applied Mathematics. Stiefel's declared goal was to advance numerical analysis. Accordingly he was looking for a way to gain access to computing power beyond the level that could be performed by simple desktop calculators.

Note: In [ZUSE47] Konrad Zuse wrote the following statement related to computers in the future: "Die in diesem Zusammenhang auftretenden Probleme sind so umfangreich, daß eine Generation von Wissenschaftlern, Technikern und Wirtschaftlern erforderlich sein wird, um sie erschöpfend zu bearbeiten."

Translation: The problems in this context are so extensive, that a generation of scientists, engineers and economists will be necessary, to process them sufficiently."

By "problems" my father meant the introduction of computers in science and companies. His plan was to build logistic computers. These computers were extended algebraic computers, like the Z4, in order to solve problems of the engineers. These problems should be formulated in the Plankalkül, which today is accepted as the first programming language of the world. Already in 1943 he wrote, that the problems of algebraic computers are solved.

Stiefel quickly realized that commercial punched card machines were not suited for mathematical work, and that the electronic computer projects under way, mainly in the US, but also in Britain, would not fill the gap for several years to come. He thus decided that ETH should build his own electronic computer. For this purpose, he delegated two of his assistants, Heinz Rutishauser and Ambros Speiser, to visit US. Their assignment was to study the state-of-the-art in US computing and then to start a suitable project at the ETH.

Rutishauser and Speiser spent most of 1949 at Havard with Howard Aiken and at Princeton with John von Neumann, but they also visited a number of other computer installations, including the ENIAC at Aberdeen and the MARK II at Dahlgren. In 1949, Stiefel was informed about the existence of the Z4 and he was told that the machine may possibly be for sale. To this day it is not clear from whom he had heard about the Z4 in Hopferau.

Figure 7-16 shows Konrad Zuse's workshop in Hopferau (Allgäu) in 1947, where he installed the Z4. The Z4 stood at the right-hand side of the workshop. Figure 7-17 shows the Z4's input and output devices and the Planfertigungsteil (program construction unit) in Hopferau. And Figure 7-18 shows Prof. Eduard Stiefel from the ETH-Zürich, who purchased the Z4 for 50,000 DM and transported it from Hopferau to Zürich in 1949.

Figure 7-16. Konrad Zuse's workshop in Hopferau (Allgäu) in 1947.

Figure 7-17. The Z4's input and output devices.

Figure 7-18. Prof. Eduard Stiefel from the ETH-Zürich.

Prof. Stiefel inspected the Z4 computer in Hopferau in 1949. Konrad Zuse explained the specifications of the Z4, which were very convincing. Stiefel then gave Konrad Zuse a differential equation to solve. My father formulated the program for the Z4, created a punch tape, and the Z4 began to work. The result was correct. Thus, despite the fact that the Z4 was only barely in operational condition, Stiefel decided that the idea of transferring it to the ETH Zürich should by all means be studied.

Stiefel wrote a letter to Rutishauser and Speiser who were in Havard at this time. Although Aiken dissuaded Speiser and Rutishauser from a relay machine (he proposed to build and use electronic machines) Rutishauser and Speiser wrote to Stiefel that the idea of taking the Z4 should not be flatly rejected. Stiefel also persuaded the ETH President, Hans Pallmann, to make the necessary funds available for renting the Z4. Stiefel must therefore receive credit for a wise decision [SPEI98], because there were enough reservations that have could made him shy away from this step. Among these reservations were:

• Relays were a technology of the past, electronics could do the job 100 or even 1000 times faster, as Aiken stated.
• The condition of the Z4 and its demonstration to Stiefel were not fully convincing. It was uncertain whether the small group around Zuse would be able to bring the machine up to a reliable operating level.
• The mechanical memory was most unusual and nobody had ever seen anything similar. An informed prediction of the Z4's operating performance seemed to be impossible. A failure of the memory would have rendered the entire machine useless.
• Installation and operation of the Z4 would absorb all the efforts of Stiefel's small group.

Ambros Speiser, who was a scientist working with the Z4 in Zürich in 1950, said in an interview to Horst Zuse in 1998 [ZUSE98]: "Prof. Stiefel did not want to build computers; he wanted to use them in an easy way for complex numerical calculations."

For this reason the speed of the computer was not the most important thing. Amongst other aspects, the simplicity of programming was very important to Stiefel. The Z4's attached Planfertigungsteil (program construction unit) convinced him to take the Z4, because it allowed scientists how to enter programs in less than three hours. Also the powerful arithmetic unit with its exception handling capability was an ideal prerequisite for the calculations Stiefel wanted to make.

In 1948, representatives from IBM visited Konrad Zuse in Hopferau. However, they were more interested in my father's patents than in his future ideas. IBM did not want to guarantee Konrad Zuse that he could work on computing in the future. For this reason only a short option contract was concluded between Konrad Zuse and IBM.

In Part 8 we will consider the innovative Zuse KG company (see also the Main Index for a quick and easy way to navigate the entire article).