Please answer the following question: Given the following passage  "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 These devices had a low operating speed and were eventually superseded by much faster all-electric computers, originally using vacuum tubes.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.",  answer the following question. Note that the answer is present within the text.  Question: What was included in the next wave of computers?
A:
vacuum tubes