A clock is an instrument for measuring time.Those used for technical purposes, of extremely high accuracy, are sometimes called chronometers. A portable clock is called a watch. The clock in its most common modern form displays the hours, minutes, and sometimes seconds that pass over a twelve or twenty-four-hour period.
The world's first self-striking clock was said to be invented by Chang Yeong-Sil, a chief enginner of Korea, in Korea during the Joseon Dynasty. It was called Chagyongru, which means "self-striking clock" in Korean
The development of electronics in the twentieth century led to clocks with no clockwork parts at all. Time in these cases is measured in several ways, such as by the behaviour of quartz crystals, or the decay of radioactive elements. Even mechanical clocks contain since come to be largely powered by batteries, removing the need for winding.