Please answer the following question: I know that the answer to the question "What was included in the next wave of computers?" is in "Early digital computers were electromechanical; electric switches drove mechanical relays to perform the calculation. These devices had a low operating speed and were eventually superseded by much faster all-electric computers, originally using vacuum tubes. The Z2, created by German engineer Konrad Zuse in 1939, was one of the earliest examples of an electromechanical relay computer.". Can you tell me what it is?
A:
vacuum tubes