A clock is a measuring instrument
A clock is a device that lets you see what time it is. Before there were clocks the time was measured warning The position of the sun, Moon and stars (and in a later time a sundial). In the 12th and 13th centuries the mechanical clocks as we know the now in monasteries, however, it was only a clockwork and had only one hand. The oldest known timepiece is a timepiece in the cathedral in Salisbury, this clock dates from 1386. In The 16th century there was a clock with a second hand, but they were very inaccurate and sometimes walked for hours. The first precise clock (accurate to the minute) was a pendulum clockwork by Christiaan Huygens (around 1655). The electric clock was created in 1841 and was built by Alexander Bain. The most accurate clock is the atomic clock, based on the definition of the second.
How does a clock work? [YouTube] uGcoIue1Bs8 [/youtube]