1940: Complex Number Generator

An attendant sends messages to the Model I relay computer.
An attendant sends messages to the Model I relay computer.

The first demonstration of remote computing took place on Sept. 11, 1940. George Stibitz, of AT&T's Bell Telephone Laboratories, demonstrated his Complex Number Calculator, the world's first electrical digital computer, to the American Mathematical Society at a meeting at Dartmouth College in Hanover, New Hampshire A teletypewriter was installed in a lecture hall at Dartmouth and connected via a modified teletypewriter line to Stibitz's electromechanical computer in New York. An attendant at the keyboard entered equations suggested by meeting attendees. The messages traveled down the circuit to New York, and the answers were returned to the teletypewriter via the same route.

What did the "brain" of the Complex Number Calculator consist of?