When I was at school we all had to use fountain pens - and the brand most people used (parker??) had cartridges that had an “extra” bit of ink in the bottom of the cartridge. When you ran out of ink you opened the pen up, tapped the end of the cartridge and some extra/ spare ink was released.
Seems pretty daft in retrospect.
Feel free to weigh in on this and general fountain pen chat - I remember the lefties getting a pretty raw deal from the mandatory fountain pen rule