hi there its quite difficult to explain it in depth due to the way i see it but i will try.... here goes....
Imagine having 2 containers 1 filled with oil the other empty on a table side by side....
right now if i put a pipe between the two containers.......
the oil will flow untill it is equal on both sides
so we use this to maintain a constant level in our smaller burner tube,the sump as you will see hold lots more oil than the burner tube so the level take a long time to change, and the ball cock keeps this level constant, much like the level of water in a toilet cistern...
ok now the burner
the oil level is kept contantly 1 mm ove the top of the dome headed nut (you could use a brass doorknob or anything smooth and with a curve this lets the oil cover the hole easily once the oil have been atomised and blown up in the burner tube...