Kids Research Express

Watch, small, portable timepiece usually designed to be worn on the person. Other kinds of timepieces are generally referred to as clocks. At one time it was generally believed that the first watches were made in Nuremburg, Germany, c.1500. However, there is now evidence that watches may have appeared at an earlier date in Italy. Early watches were ornate, very heavy, and made in a variety of shapes, e.g., pears, skulls, and crosses; the faces were protected by metal latticework. Watch parts were made by hand until c.1850, when machine methods were introduced by watch manufacturers in the United States. The introduction of machine-made parts not only cut manufacturing costs but increased precision and facilitated repairs. To insure the accuracy of a watch over a long period, bearings made of jewels (usually synthetic sapphires or rubies) are utilized at points subject to heavy wear. The mechanical watch contains a mainspring to drive the watch's mechanism. Part of the mechanism inc

Clepsydra (klĕp`sĭdrə) or water clock, ancient device for measuring time by means of the flow of water from a container. A simple form of clepsydra was an earthenware vessel with a small opening through which the water dripped; as the water level dropped, it exposed marks on the walls of the vessel that indicated the time that had elapsed since the vessel was full. More elaborate clepsydras were later developed. Some were double vessels, the larger one below containing a float that rose with the water and marked the hours on a scale. A form more closely foreshadowing the clock had a cord fastened to the float so that it turned a wheel, whose movement indicated the time. A further step was the use of gear wheels and a turning pointer. It is believed that clepsydras were used in Egypt c.2000 B.C.; from Egypt they were introduced into Greece and later from there into Rome.

Hourglass, glass instrument for measuring time, usually consisting of two bulbs united by a narrow neck. One bulb is filled with fine sand that runs through the neck into the other bulb in an hour's time. The date of its invention is unknown, but it was in use in ancient times. Similar devices for marking shorter periods of time, e.g., three-minute sandglasses for timing the cooking of eggs, are still used occasionally.

Sundial, instrument that indicates the time of day by the shadow, cast on a surface marked to show hours or fractions of hours, of an object on which the sun's rays fall. Although any object whose shadow is used to determine time is called a gnomon, the term is usually applied to a style, pin, metal plate, or other shadow-casting object that is an integral part of a sundial. Forerunners of the sundial include poles or upright stones used as gnomons; pyramids and obelisks were so used in Egypt. Both stationary and portable sundials were probably developed in Egypt or in Mesopotamia. The earliest extant sundial, an Egyptian instrument of c.1500 B.C., is a flat stone on which is fixed an L-shaped bar whose short vertical limb casts a shadow measured by markings on the longer horizontal limb. The sundial was greatly improved (c.1st cent. A.D.) by setting the gnomon parallel to the earth's axis of rotation so that the apparent east-to-west motion of the sun governs the swing of the

Clock, instrument for measuring and indicating time. Predecessors of the clock were the sundial , the hourglass , and the clepsydra . See also watch . The Evolution of Mechanical Clocks The operation of a clock depends on a stable mechanical oscillator, such as a swinging pendulum or a mass connected to a spring, by means of which the energy stored in a raised weight or coiled spring advances a pointer or other indicating device at a controlled rate. It is not definitely known when the first mechanical clocks were invented. Some authorities attribute the first weight-driven clock to Pacificus, archdeacon of Verona in the 9th cent. Gerbert, a learned monk who became Pope Sylvester II, is often credited with the invention of a mechanical clock, c.996. Mechanical figures that struck a bell on the hour were installed in St. Paul's Cathedral, London, in 1286; a dial was added to the clock in the 14th cent. Clocks were placed in a clock tower at Westminster Hall, London, in 1288 and i

In the life sciences, evidence has been found that many living organisms incorporate biological clocks that govern the rhythms of their behavior (see biological  rhythm ). Animals and even plants often exhibit a circadian (approximately daily) cycle in, for instance, temperature and metabolic rate that may have a genetic basis. Efforts to localize time sense in specialized areas within the brain have been largely unsuccessful. In humans, the time sense may be connected to certain electrical rhythms in the brain, the most prominent of which is known as the alpha rhythm at about ten cycles per second.

In addition to relative time, another aspect of time relevant to physics is how one can distinguish the forward direction in time. This problem is apart from one's purely subjective awareness of time moving from past into future. According to classical physics, if all particles in a simple system are instantaneously reversed in their velocities, the system will proceed to retrace its entire past history. This property of the laws of classical physics is called time reversal invariance (see symmetry) ; it means that when all microscopic motions of individual particles are precisely defined, there is no fundamental distinction between forward and backward in time. If the motions of very large collections of particles are treated statistically as in thermodynamics , then the forward direction of time is distinguished by the increase of entropy , or disorder, in the system. However, recent discoveries in particle physics have shown that time reversal invariance is not valid even on the