A pen that carries a supply of ink inside it is called a fountain pen. This ink supply is in a reservoir, either a disposable cartridge or a rubber, sac-like container inside the pen’s barrel.
Disposable cartridges can be removed completely and replaced with a new one when they run dry, but the sac is filled by a different method. On the side of the pen’s barrel is a lever. When you pull out on the lever, its other end pushes inside the pen and presses against the sac. This forces out the air from the empty sac.
Then when you place the tip, or nib, of the pen into a bottle of ink and release the lever, the ink is drawn up through tiny tubes into the sac. This is called the vacuum method, since the vacuum inside the sac draws up the ink.
The first working fountain pen, developed in 1884 by Lewis Waterman, had to be filled with ink from an eyedropper!