Measurement is the process or the result of determining the magnitude of a quantity, such as length or mass, relative to a unit of measurement, such as a meter or a kilogram.

The word measurement stems, via the Middle French term mesure, from Latin mēnsūra, and the verb metiri.

The science of measurement is also called the field of metrology.

A typical tape measure with both metric and US units and two US pennies for comparison

Control theory is an interdisciplinary branch of engineering and mathematics, that deals with the behavior of dynamical systems. The desired output of a system is called the reference. When one or more output variables of a system need to follow a certain reference over time, a controller manipulates the inputs to a system to obtain the desired effect on the output of the system

The concept of the feedback loop to control the dynamic behavior of the system: this is negative feedback, because the sensed value is subtracted from the desired value to create the error signal which is amplified by the controller.

A watch is a timepiece that is made to be worn on a person. It is usually a wristwatch, worn on the wrist with a strap or bracelet. In addition to the time, modern watches often display the day, date, month and year, and electronic watches may have many other functions.

Most inexpensive and medium-priced watches used mainly for timekeeping are electronic watches with quartz movements. Expensive, collectible watches valued more for their workmanship and aesthetic appeal than for simple timekeeping, often have purely mechanical movements and are powered by springs, even though mechanical movements are less accurate than more affordable quartz movements.

Before wristwatches became popular in the 1920s, most watches were pocket watches, which often had covers and were carried in a pocket and attached to a watch chain or watch fob. Watches evolved in the 17th century from spring powered clocks, which appeared in the 15th cen

Different kinds of movements move the hands differently as shown in this 2 second exposure. The left watch has a mechanical 21,600 bph movement, the right one has a quartz movement.

A Russian mechanical watch movement.

Vector clocks is an algorithm for generating a partial ordering of events in a distributed system and detecting causality violations. Just as in Lamport timestamps, interprocess messages contain the state of the sending process’s logical clock. A vector clock of a system of N processes is an array/vector of N logical clocks, one clock per process; a local "smallest possible values" copy of the global clock-array is kept in each process, with the following rules for clock updates:

Example of a system of vector clocks

• Initially all clocks are zero.
• Each time a process experiences an internal event, it increments its own logical clock in the vector by one.
• Each time a process prepares to send a message, it increments its own logical clock in the vector by one and then sends its entire vector along with the message being sent.
• Each time a process receives a message, it increments its own logical clock in the vector by one and updates each element in its vector by taking the maximum of the value in its own vector clock and the value in the vector in the received message (for every element).