Mach meter dial, Type K B 222/01, c 1950s.
© Science Museum / Science & Society Picture Library
This device is used to measure the velocity of an aircraft relative to the speed of sound. Mach 1 is reached when an aircraft attains a speed of 331 metres (1087 feet) per second and thereby 'breaks the sound barrier'. The exact speed varies with altitude. The sound barrier was first broken by the American Bell X-1 aircraft piloted by Chuck Yeagher in 1947.