A short circuit being an unexpected jump of electricity that bypasses its intended route, a long circuit is just the opposite- An unexpected detour of electricity that takes longer to make it to its intended destination.
To qualify as a long circuit, the electrical impulse must still arrive and complete its journey, but must in the process be delayed by an alternative route to the circuit as designed, usually involving departure and then reintegration into the original circuit. A common example is when a person or animal touches an active circuit, shocks themself, and recoils in pain or dies while the circuit goes about its business and functions normally, albeit with a slight delay.
The longest circuit recorded happened in late 1968 during the launch of the Apollo 8 mission. A predecessor to the moon landing, Apollo 8 orbited the moon before returning to Earth. It was crewed by three men, or so NASA thought. But in fact they had a stowaway.
NASA used to employ hundreds of safety and monitoring measures on every mission, owing to the importance, expense and danger of their activities. One such measure was a circuit that connected one fin of the Saturn V rocket to the gantry, assuring engineers that the rocket was in place on the launch pad. Once launched, the circuit would be broken and the location diode in Launch Command would turn on. However, with Apollo 8, the final circuit impulse before launch happened just as the rocket lifted off, meaning the circuit was never completed, and the rocket didn’t log as having been launched.
This was no problem of course, the launch went well and anyone could plainly see that the rocket had gone up. But nonetheless, no diode activated. The electrical impulse was, of course, stuck in the Saturn V. As each section of the launch vehicle was jettisoned, the impulse stayed as close to the safety of the crew module as it could. The circuit thus unintentionally extended, for a few days, all the way to the moon, several orbits thereof, and a return to Earth where it landed with the splashdown of the crew, transited the ocean and made its way back to launch command.
So it was that the launch diode finally lit up one week and half a billion miles later, making it the longest circuit ever, at least until I tried to render a video in Premiere this morning and it’s still fucking processing.