Z2
Zuse was unsatisfied with the reliability of the binary switching metal sheets used in the Z1, Zuse next constructed the Z2 computer. The Z2 used the same mechanical memory as the Z1, but he used 800 old relays from phone companies to construct the arithmetic and control units. The Z2's arithmetic unit consisted of a 16- bit fixed-point engine, because he wanted to test the reliability of relays for arithmetic calculations. Unfortunately, photos and plans of the Z2 were destroyed by allied air raids during the war. But, the Z2 later led to the Z3 completely out of relays 600 for arithmetic unit, 1800 for the memory and control units.