The bug at Harvard was very literal since it was a moth trapped between the points inside an electromagnetic relay. Its presence led to problems in the functioning of the whole computer which were resolved when the moth was removed. The relays inside the Mark II were used for logic calculations using the Boolean system, but if the relay didn’t switch in the correct way the algebraic function would be incorrect.
Development of the Harvard Mark II was financed by the United States Navy, which explains the involvement of Grace ‘Amazing Grace’ Hopper. She had previously been assigned to work as one of the very first programmers of the Harvard Mark I during the Second World War when she was a member of the US Navy.
Urban legends suggest that the discovery of the moth inside the Mark II caused the use of the terms ‘bug’ and ‘debugging’ in computer programming. However, the word ‘bug’ is known to have been used to describe malicious beasties since medieval times, and was well-known in the world of electrical engineering since at least the time of the telegraph. Even Hopper, who is credited with popularising the term ‘debugging’ to find problems in computer code, regularly stated that the moth simply fitted with existing terminology.
The preservation of the moth in the log-book of the Mark II alongside the label ‘First actual case of bug being found’ is further proof that computer scientists were already aware of the term ‘bug’ to describe a fault in a computer.
