Unlike ordinary ball pens which rely on gravity to feed ink, the Space Pen® writing instrument has a replaceable cartridge which is pressurized. At nearly 35 pounds per square inch, ink is continuously fed to the tungsten carbide ball, allowing the user to write at any angle, even upside down. All metal , 10 mm (.380 inch) diameter with side button mechanism. Contains a #PR4 Black Ink, Medium Point Pressurized Fisher Space Pen Refill. Gift boxed.