I have operated carbons only a few times. The guy who showed me even demonstrated how to change carbon rods without extinguishing the arc. (Joinable carbons.) I would never do that! 100 amps DC and 3,000º C! I ain't going near that while it's live! This guy was an old-time operator. That's the way they did things, then. A good operator will know how fast his carbons burn and how much film he has left. He'll make sure he has enough "stick" in the projector before he starts.