The Harvard Mark I (officially the IBM Automatic Sequence Controlled Calculator, or ASCC) was one of the first large-scale automatic digital computers. Completed in 1944, it represented a crucial step from calculating machines to true computers and served as a training ground for pioneering programmers including Grace Hopper.
Design and Construction
The Mark I was designed by Howard Aiken of Harvard University and built by IBM. It was a massive electromechanical machine[1]:
- 51 feet long, 8 feet tall, weighing 10,000 pounds
- 765,000 components connected by 500 miles of wire
- Used mechanical relays rather than electronic vacuum tubes
- Could perform 3 additions per second (slow by later standards)
The machine read instructions from punched paper tape, making it programmable—a key distinction from earlier calculating machines.
Wartime Service
The Mark I was immediately pressed into service for the U.S. Navy during World War II. It calculated ballistic tables, naval gun trajectories, and other military computations. Later, it performed calculations for the Manhattan Project’s implosion lens design[2].
Grace Hopper and the First Manual
Lieutenant Grace Hopper joined the Mark I team in 1944 as one of its first programmers. She wrote the 561-page operating manual, one of the earliest comprehensive programming guides ever created[3].
Programming the Mark I meant:
- Punching instructions onto paper tape
- Manually sequencing complex calculations
- Debugging by inspecting mechanical components
This experience led Hopper to envision easier ways to program computers, eventually inspiring her to create the first compiler.
The First Computer Bug
In 1947, operators found a moth trapped in a relay of the Mark II (a successor machine), causing a malfunction. Though the term “bug” for technical problems predates this, Hopper and her colleagues taped the moth into the logbook with the note “First actual case of bug being found”—popularizing the term in computing.
Legacy
The Harvard Mark I bridged the gap between mechanical calculators and electronic computers:
- Demonstrated that large-scale automatic computation was practical
- Trained the first generation of computer programmers
- Established Harvard’s role in computing history
- Led to the Mark II, III, and IV series of computers
The machine operated until 1959 and is now partially displayed at Harvard’s Science Center.
Sources
- Computer History Museum. “The Mark I.” Technical specifications and history.
- Wikipedia. “Harvard Mark I.” Wartime applications and Manhattan Project work.
- Yale University. “Biography of Grace Murray Hopper.” Hopper’s role in programming the Mark I.