Ballpoint pens are used as a writing instrument that replaced quill pens and fountain pens during the mid-nineteenth century. Most ballpoint pens are relatively inexpensive thanks to their popularity and the materials used.
A ballpoint pen is formed of an ink-filled chamber capped by a small sphere. When pressure is applied the ink flows forward and is stopped by the ball. When the pen is moved, the ink slides around the ball and leaves an even mark on paper.
Ballpoint pens have two types: refillable and disposable. Refillable ballpoint pens can be refilled with ink. Disposable ballpoint pens are useless once the ink has run out or becomes dry. Because of environmental concerns, many people prefer refillable ballpoint pens as they produce less waste and save money.
Space pens are ballpoint pens that use a more free-flowing ink than normal ballpoints, and they also include a small piston that forces the ink toward the ball. This is why space pens can write in zero gravity and upside down, since the ink is unable to retract back into the chamber.
Rollerball pens use a gel-based ink instead of an oil-based ink that most ballpoints use. This results in a more fluid flow of ink and a larger diversity of color.
The ball of a ballpoint pen is made of tungsten carbide, a material that resist deformity under great pressure.