The Life and Work of Konrad Zuse (by Horst Zuse) Part 6: 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 43b shows the Methfesselstraße 7 and 10, where Konrad Zuse built the Z3 and started the construction of the Z4.
 Fig.43b (Left). The 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). Fig.44 (Right). 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.
 Fig.45 (Left). The Z4's input and output devices, as drawn in 1942 by Konrad Zuse. Fig.46 (Right). 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 (Fig.47 and Fig.48), and the increasing difficulty of living in Berlin (which worsened daily) made it impossible to finish the Z4 completely.
 Fig.47 (left). The area around the Brandenburger Tor (Brandenburg Gate) in 1945. Fig.48 (right). The Brandenburger Tor in 1947.