In 1937, George Stibitz decided that the electromechanical
relays that were the chief components in telephone switching systems could be
used for another purpose. From the relays, flashlight bulbs, and a switch made
from a tobacco tin, he built the first binary adder.
In 1939 , Stibitz and S.B. Williams built the Complex Number Calculator, the world's first electrical digital computer. Its brain consisted of 450 telephone relays and 10 crossbar switches, and it could find the quotient of two eight-place complex numbers in about 30 seconds. Three teletypewriters provided input to the machine.
In 1940, Stibitz took one of the teletypewriters to an American Mathematical Association meeting at Dartmouth, New Hampshire, and used it to communicate over phone lines with the Complex Number Calculator in New York. This was the world's first demonstration of remote computing.
The CNC, later renamed the Model 1 Relay Computer, remained in operation until 1949.