You don't mention the make of the iron but no matter...
The steel ball might possibly be to "hold" a control wheel in position and give "feel" to it. A soft click click as you turn the heat up or down. Ball will fit into grooves or slots in plastic or nylon wheel - spring will press ball against wheel...maybe. Could be controlling steam like a pressure release valve in which case ball will cover a hole and spring will press against ball - but I doubt it.
Peccavi
October 2008