The Aberdeen chronograph was the first portable gun chronograph, an instrument for measuring the muzzle velocity and striking power of a projectile fired by a gun. It was invented in 1918 by Alfred Lee Loomis at the U.S. Army's Aberdeen Proving Ground. The method prevalent at the time was the Boulengé chronograph, which relied on the projectile passing through two wire screens. Breaking the first screen would release a rod held by electromagnets. While the rod was free-falling, breaking the second screen would activate a knife that marked the rod.