The Roentgen meter was a meter on board the DeLorean time machine that measured the amount of radiation in Roentgens produced by the plutonium powering the flux capacitor. When the plutonium chamber ran out of fuel, an alarm sounded indicating refueling.
This occurred to Marty McFly after he arrived in 1955 without any more plutonium. Coincidentally, the engine also ceased.
The Roentgen meter was one of the many items that Emmett Brown had annotated with Dymo labels.