I The first generation computers were huge, slow, expensive, and often undependable. In 1946
two Americans, Presper Eckert, and John Mauchly built the ENIAC electronic computer which used vacuum tubes instead of the mechanical switches of the Mark I. The ENIAC used thousands of vacuum tubes, which took up a lot of space and gave off a great deal of heat just like light bulbs do. The ENIAC led to other vacuum tube type computers like the EDVAC (Electronic Discrete Variable Automatic Computer) and the UNIVAC I (UNIVersal Automatic Computer).
The vacuum tube could be used to make binary. If the tube was "on" (powered) it was a 1, if it was "off" (no power) it was a zero. It was sort of like computing with light bulbs! Unlike light bulbs, they gave of no light, and so if one burnt out, it could take weeks to find.
The ENIAC gave off so much heat that they had to be cooled by gigantic air conditioners. However even with these huge coolers, vacuum tubes still overheated regularly. It was time for something new.