On This Day in Tech: September 9, 1947
On September 9, 1947, operators working with the Harvard Mark II electromechanical calculator traced a malfunction to a moth caught in relay number 70 on panel F. They removed the insect, taped it into the machine’s logbook, and added the joking note, “First actual case of bug being found.” The preserved page turned a routine maintenance incident into one of computing’s most recognizable artifacts. Its appeal comes from the literal collision between a living bug and the relay hardware that controlled the machine’s calculations.
The Mark II was a room-sized calculator built at Harvard for the U.S. Navy during an era of intensive military computation. Instead of the fully electronic circuits associated with later computers, it relied heavily on thousands of electromagnetic relays—switches whose metal contacts opened and closed in response to electrical signals. Relays were dependable by the standards of the time, but dirt, wear, loose connections, and misalignment could interrupt a sequence. A moth lodged between contacts could therefore stop part of the machine just as surely as a wiring fault. The calculator performed mathematical operations automatically, yet it still depended on a human staff to prepare jobs, monitor its progress, interpret unusual behavior, and maintain the dense mechanical network. Diagnosing a failure required operators to narrow the problem to a specific circuit, panel, and component.
The famous story is often simplified. The logbook did not create the engineering meaning of “bug,” which had been used for technical defects long before electronic computing; Thomas Edison employed the term in the 19th century. Grace Hopper was a member of the Harvard computing team and later told the moth story widely, helping it become part of computer culture, but surviving evidence does not establish that she personally found the insect or wrote the entry. The careful version is still delightful: a team already familiar with the figurative word encountered the rare literal example and preserved the joke.
The incident endures because debugging remains central to every layer of technology. Modern failures are more likely to arise from software logic, timing, memory, or networks than from an insect in a relay, yet the investigative process is recognizable: observe the symptom, isolate the subsystem, identify the cause, correct it, and record what happened. The Mark II log page compresses that discipline into one tangible scene. On September 9, 1947, an ordinary repair became a lasting symbol for the patient work of making complex systems behave as intended.
It was a memorable literal bug, not the origin of the engineering term. People had used “bug” for mechanical and electrical faults decades before the Mark II incident.
The logbook’s humor depends on that earlier usage. The operators already knew the figurative term, then documented an actual insect inside the machine.
Grace Hopper worked with the Harvard computing group and later popularized the story. However, the surviving logbook does not prove that she discovered the moth or wrote the famous note.
It is most accurate to credit the Mark II operating team collectively while recognizing Hopper’s role in preserving and retelling the episode.
The calculator used electromagnetic relays as switches. If material blocked a relay’s contacts, the expected electrical connection could fail and interrupt the machine’s sequence of operations.
Operators diagnosed problems by tracing signals through panels of relays and wiring. In this case, the search led to relay number 70 on panel F.
Explore more of "On This Day ..."
Discover more events from the same date across news, politics, technology, sports, and other fields. Each link highlights significant moments that shaped history on different fronts.