Samguk Sagi records that an office overseeing the use of water clocks had been established during the Three Kingdoms Period.
The Korean water clock consisted of two stacked jars of water, with water dropping from the top to the bottom at a measured rate.
The level of the water indicated the time of the day.
This was very inconvenient because a person had to be always be on guard, so that a drum could be banged at each hour to inform the public of the current time.

Self-striking water clocks were not new, having already been invented by the Arabians and the Chinese (in 1091).
Upon hearing about the usage of self-striking water clocks in foreign countries, Sejong assigned Jang and other scientists to build a clock emulating such automatic devices.
After their initial attempts failed in developing an operational water clock, Jang traveled to China to study the various designs of water clocks.
When he returned in 1434, Jang created Korea's first self-striking water clock, the Jagyeokru (자격루), which would mark the hour automatically with the sounds of a bell, gong, and drum, and was used to keep the standard of time in the Joseon Dynasty.
This water clock was not preserved well and did not survive; however, reconstructions of the Jagyeokru based on text descriptions have been made.

Circling the clock were 12 wooden figures that served as indicators of time.
 There were four water containers, 2 jars that received the water, and 12 arrows floating inside the lower container.
As the water from the upper containers seeped down the pipe to the lower container, one of the arrows would tilt a board filled with small iron balls; a ball would roll down a pipe to a container of larger iron balls.
The collision would cause the larger balls to travel down a lower pipe and hit a giant cymbal, announcing the time to the community.
 A ball would then land on another container, which is part of a complex system of levers and pulleys that move wooden figures to indicate the time visually.
Samguk Sagi records that an office overseeing the use of water clocks had been established during the Three Kingdoms Period. The Korean water clock consisted of two stacked jars of water, with water dropping from the top to the bottom at a measured rate. The level of the water indicated the time of the day. This was very inconvenient because a person had to be always be on guard, so that a drum could be banged at each hour to inform the public of the current time.
Self-striking water clocks were not new, having already been invented by the Arabians and the Chinese (in 1091). Upon hearing about the usage of self-striking water clocks in foreign countries, Sejong assigned Jang and other scientists to build a clock emulating such automatic devices. After their initial attempts failed in developing an operational water clock, Jang traveled to China to study the various designs of water clocks. When he returned in 1434, Jang created Korea's first self-striking water clock, the Jagyeokru (자격루), which would mark the hour automatically with the sounds of a bell, gong, and drum, and was used to keep the standard of time in the Joseon Dynasty. This water clock was not preserved well and did not survive; however, reconstructions of the Jagyeokru based on text descriptions have been made.
Circling the clock were 12 wooden figures that served as indicators of time.  There were four water containers, 2 jars that received the water, and 12 arrows floating inside the lower container. As the water from the upper containers seeped down the pipe to the lower container, one of the arrows would tilt a board filled with small iron balls; a ball would roll down a pipe to a container of larger iron balls. The collision would cause the larger balls to travel down a lower pipe and hit a giant cymbal, announcing the time to the community.  A ball would then land on another container, which is part of a complex system of levers and pulleys that move wooden figures to indicate the time visually.
Jang Yeong-sil