The earliest control systems can be traced back to the float regulator mechanisms that were used in Greece around 270 BC. The need for accurate time tracking inspired the Greek water clock (clepsydra), which leveraged the simple float regulator to maintain a constant flow of water. The float regulator would maintain the water level in a primary tank at a constant depth; water kept at a constant depth maintained a constant water pressure.
Constant pressure resulted in a constant flow of water through a tube that would fill a secondary tank at a constant rate. The level of the second tank was used to measure time, which was displayed on a dial using a second float. A similar float regulator mechanism is still used in our toilets today. A construct that uses input from another device (float) to maintain a value (water level) is called a feedback controller.
The following diagram details the components of a simple Greek...
