acid

“Machine problems, called bugs, were very often caused by fraying of the brushes on the counters, which caused them to spark. When this happened, the operators would go to Hopper and borrow the little mirror from the handbag she always had with her. Then they turned the lights off and held the mirror down into the machinery to locate where the counters were sparking.”
― Kathleen Broome Williams, Grace Hopper: Admiral of the Cyber Sea