A wristwatch is a small machine that measures time ( hour ), which people use to recognize time and develops a watch that was carried in a pocket . 

Historically, the first small clock was credited with the German locksmith Peter Heinlein. In the early 16th century, Heinlein invented a key spring to power the watch. The clocks were then run by hanging weights. For the watch to work, it must remain stationary in the vertical position for the hanging weights to work. The springs have enabled watchmakers to produce small mobile watches. Watchmaking soon spread to England, France, and Switzerland.