A pen with a refillable reservoir may have a mechanism such as a piston to draw ink from a bottle through the nib, or it may require refilling with an eye dropper. A fountain pen reservoir can be refillable or disposable the disposable type is called an ink cartridge. A fountain pen nib, unlike the tip of a ballpoint, gel or rollerball pen, has no moving parts. While writing, ink is pulled out of this slit by capillary action. The feed delivers the ink to the slit in the nib. Driven by gravity, the ink flows from a reservoir to the nib through a feed, which is in general a specially shaped solid block of material with channels and grooves cut into it.
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The ball is held captive in a socket at the tip of the pen with one half exposed and the other half immersed in ink from the pen's reservoir.
![don juan font and shape cartridge don juan font and shape cartridge](https://www.mdpi.com/toxins/toxins-12-00147/article_deploy/html/images/toxins-12-00147-g001-550.jpg)
Writing tip of a ballpoint pen and 1mm white bar for scale