Second generation computers used transistors instead of vacuum tubes. First demonstrated in 1947 by AT&T’s Bell Laboratories, transistors regulate current or voltage flow and act as a switch for electronic signals. By the late 1950’s, transistors had replaced vacuum tubes as the processing and memory technology for most computers. An important development in software also differentiated second generation computers from the first generation ancestors. Second generation computers also ran programming language compilers that allowed programmers to write instructions using English-like commands rather than machine language.