What is the large metal can that I've found in my fluorescent lamp for?

your are part right the thing you found is old starter transformer more than a starter it works like the coil on old model car it increases the voltage going to the light its self making it start sooner it heats up the vapor inside of tube so that neon gases will start to glow faster than they would if it had to wit for the regular electricity to do they still do the same thing in new fixtures but it incorporated in to balanced box by the way it is not a good idea to go around plugging up things that you don't know what they do .my dad put a screwdriver in the high voltage tube of a old tv one time he woke up and hour later and had to go mama a new tv after we clean up the living room that is tv parts and other things all over the place ray smith minden louisana