The principle of the modern computer was first invented by Turing in 1935. He conceived an abstract digital computing machine endowed with an unlimited memory and a scanner which would move through the symbols stored in the memory, read them and would write further symbols. This machine is designated as the Universal Turing Machine.
During World War II, Turing worked as a cryptanalyst at the Government Code and Cypher School (GC&CS) and this is where he met Thomas Flower who was working on large scale, high speed electronic switching. Till then, most of the computing devices were electromechanical i.e. their small components were electrically driven and had mechanical switches called relays. Flower was the first person who made extensive use of vacuum tubes for digital data processing. He used this device for controlling the connections between telephone exchanges but never really thought of using electronic equipments for numerical calculations.
Atanasoff was perhaps the first individual who (during 1937 - 42) developed technique for utilizing vacuum tubes to perform numerical calculations digitally. He and his pupil Clifford Berry devised a small-scale special purpose electronic digital machine, which contained 300 vacuum tubes. Though the electronic part of Atanasoff-Berry computer functioned properly, the device as a whole was not quite successful.