The first design for a programmable computer one that would follow a set of instructions is usually considered to be the "Analytic Engine" invented by English inventor Charles Babbage in 1832. His device was designed to perform a sequence of calculations using instructions input on punched cards, and it included a memory "store" and a processing unit. It was entirely mechanical in design. Unfortunately, Babbage never assembled his computer, and it was not until the 1900s, with the invention of the electron tube, that components for a viable electronic computer became available. An electron tube is a device that can block, amplify, or act as an on/off switch for an electric current. During the 1920s and 1930s, scientists investigated how to link these devices in arrays that would accept electric signals representing numbers, process the signals according to programme, and output the results. Whereas electron-tube computers were huge, and programming them involved changing their circuitry by plugging and unplugging cables.