Much of the hazard in catapulting airplanes from ships is eliminated by a speed indicator that shows how fast the skid travels so proper adjustment may be made. The tester, designed in General Electric laboratories, includes two coils mounted afoot apart at the end of the catapult runway and a permanent magnet attached to a skid on the catapult. As the skid, carrying its magnet, whizzes past the two coils, voltage impulses are set up first in one coil and then in the other. The time interval between these two impulses is registered by a needle on a connected instrument indicating the rate of speed, which should be great enough to hurl the plane into the air, yet slow enough to avoid injury to pilot, and unnecessary strain on the plane.