A small rotating ball constructed out of steel, brass or tungsten carbide disperses ink through the ink-filled tube--called the ink reservoir-- located on the inside of the pen as the pen pushes it onto the paper, according to "Made in the USA" author Mindi Rose Englart. The pen dispenses ink or a colored fluid designed for drawing or writing. Ballpoint pens generally evenly distribute the fluid, resulting in an even-handed document or drawing.
Writing instruments preceding the ballpoint pen commonly distributed the ink or writing fluid unevenly. Quill and fountain pens proved untidy and difficult to work with. When the liquid dried up in these antiquated instruments, they had to go through a meticulously messy cleaning to become usable, says Russel C. Alexander, author of "I Thought of It First."
The key to the pen's design lies in the ball. It provides a buffer between the surface you are writing or drawing on and the quick-drying ink inside. The ball freely rotates while ink continuously feeds through the reservoir. The liquid rolls onto the ball, allowing the ink to flow onto the page. The rolling mechanism allows the fluid to roll on both the ball and on the paper while simultaneously sealing it from air to prevent the fluid from drying out in the reservoir, according to Ideafinder.com.
The pen's socket keeps the ball in place in the unit. A tight socket grants the ball enough room to freely roll around. Gravity forces the ink adhering to the ball into the reservoir where it then bleeds onto the paper or writing surface.
The width of the ball determines the size of the line drawn. A "point seven" millimeter pen produces a 0.7 mm line. Ballpoint pens come as small as 0.1 mm pens, which are called ultra-fine ballpoint pens. The ink, usually blue or black, also comes in other colors such as red, green and pink. Ink is made of pigment or dye dispersed in a liquid called the vehicle.