The first real progress toward electronic digital computer came in the late 1930s when Howard Aiken of Harvard University and George Slibits of Bell Telephone Laboratories developed an automatic calculator using relay networks;the relay is an electromagnetically controlled switch.Other relay machines were developed during World War II for artillery ballistic calculations.Although these machines were relatively slow and comparatively large. they demonstrated the versatility of the electronic computer. Then,in the early 1940s, John Mauchly and J.Presper Eckert,Jr.,of the University of Pennsylavania designed and built a vacuum tube computer,which they called the electronic numerical inegrator and calculator(ENIAC);it was completed in 1945 and installedat Aberdeen Proving Ground,Maryland. ENIAC used 18000 electron tubes,which required tremendous amounts of power;its failure rate wa high and it was difficult to program because a plugboard was required.
Three very important discoveries were then made,which began the rapid evolution toward today's digital computer. First,John von Neumann proposed that the program reside in the computer's memory where it could be changed at will,solving the pogranmming difficulties of ENIAC;second,in 1947 the transistor was invented by John Bradeen,Waltr H.Brattain, and William Shockley,which drastically reduced the size and power requirements by replaceing the electron vacuum tube;and.third, J.W.Forrester and his associates at the Massachusetts Institute of Technology developed the magnetic,core memory,which made large amount of storage feasible.


Reference book:
Digital Logic Circuit Analysis and Design
(Authors:Victor P.Nelson
H.Troy Nagle
J.David Irwin)