English: Schmidt K5 piston-style standard international size fountain pen converter containing a user inserted 2.5 mm diameter Marine grade 316 stainless steel bearing ball. Converters are intended to be filled and refilled with bottled fountain pen ink. They are an alternative for using disposable ink cartridges. Fountain pen inks feature differing surface tensions that can cause an ink to adhere or "stick" against the inside of the reservoir. Common solutions for this problem are adding a small (rust-proof) ink agitating object like a bearing ball, short spring or hollow tube in the tubular reservoir to mechanically promote free movement of the contained ink and ink/air exchange during writing. The 316 stainless steel ball moves around through ink in the converter with ease. Adding a 2.5 mm diameter sphere translates to sacrificing 8.18 microliter or 0.00818 milliliter ink capacity. The 2.5 mm diameter sphere weighs 65.44 milligram (316 stainless steel density ≈ 8 g/cm3)
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