Of course, the second-simplest possible system for delivering ink to a fountain pen is to provide a sealed cartridge that is pre-filled with ink. (The simplest method is to buy a pre-filled disposable fountain pen, however, this is not necessarily the wisest choice.) When inserted into the pen, a seal is broken, thus providing a path for the ink to flow from the inside of the cartridge to the nib.
This concept is commonly thought to have been introduced by Waterman with their C/F (Cartridge Filler) model, introduced in 1953 and finally patented in 1957 (US Patent 2,802,448), however there were examples that preceded it such as the LUS Atomica pen from Italy (with a plastic cartridge) and the Eagle Pencil Company’s glass-cartridge example from 1890.
Many, if not most, fountain pens today can use a cartridge or a fillable converter, however, one has to be somewhat careful. Although many are designed to use the standard-sized “International” cartridge, there are other proprietary sizes that are mutually incompatible. To make things even more slightly confusing, some manufacturers design their pens to use the proprietary cartridges from another manufacturer: as only one example, Aurora pens are compatible with the Parker cartridges. Finally, some manufacturers have changed their size over time: for example, a modern Waterman cartridge will not fit a vintage C/F model.
Converters
It isn’t uncommon for a first-time buyer of a fountain pen to choose a cartridge-based model due to the simplicity and cleanliness of the filling system. However, as they progress, the attraction of bottled inks with a much wider range of colours and options, takes hold. The expensive solution to this new problem is to buy a second pen. However, it is not uncommon for a second fountain pen to be merely the first of a perpetually-expanding collection, which makes the solution much more expensive than anticipated.
A cheaper option is to replace the disposable cartridge with a refillable version, called a converter. Just as there are with fountain pens, there are various filling systems used in converters. The two most popular are 1) a pressure bar with an ink sac inside a housing and 2) a piston filler, however, there are other options such as a pump mechanism. Examples of these are shown below.
Fountain Pen Design 