Encyclopedia of popular science knowledge: "Time Knowledge"

Chapter 2 All kinds of clocks (2)
The travel time of mechanical clocks often varies due to changes in temperature and the like.Generally speaking, when the temperature is high, the travel time is slow, and when the temperature is low, the travel time is fast.Now, in government agencies, factories and schools, mechanical clocks have been replaced by electric clocks, because electric clocks driven by electric current have simpler structures and more accurate timekeeping.

A clock powered by electricity is called an electric clock.The simplest electric clock driven by alternating current is an electric motor with simple structure.It drives a series of gear transmission devices to drive the hands to indicate the time.

There is a clock in Beijing Railway Station called "Sub-Mother Clock". There is a mother clock and a child clock.The pendulum of the master clock swings back and forth continuously, sending current outward every 1 minute.When the sub-clock receives a pulse current, the mechanical device will drive the minute hand to jump up one grid.It does not need to install huge pendulums and springs, and its power is replaced by electricity.

In 1952, the United States invented the electric watch, which used chemical batteries as power to replace the spring in mechanical watches.The energy provided by the chemical battery is relatively stable, so the accuracy of the travel time has been improved.

However, since the electric energy of the battery is transmitted to the balance wheel through the mechanical contacts, and the mechanical contacts are switched many times, it is easy to be damaged.This table could not be promoted.However, it pointed out the direction for watches, which were powered by batteries, and soon electronic watches were born and developed rapidly.

After the invention of semiconductors, electronic watches have experienced four generations of evolution.

In 1963, Switzerland developed the "balance wheel hairspring electronic watch", which is the first generation.

This is different from electric watches in that transistors, resistors and other components are used to form non-contact switching circuits to replace fragile mechanical contacts.It does not need a mainspring, the gear system has less force and less wear, so it has a long service life and its travel time accuracy is slightly higher than that of electric watches. It was popular in the world market in the 60s.

Another kind of tuning fork type electronic watch is also powered by batteries.It uses a tuning fork oscillation system composed of a small tuning fork and a crystal triode non-contact switch circuit to replace the balance spring vibration system.It turns out that as long as the tuning fork is tapped lightly, the tuning fork will vibrate and emit a sound of a certain frequency.This singing "pendulum" sang the beautiful song of time with the accompaniment of capable electronic circuits.The tuning fork vibrates at a frequency of 300 hertz per second, making a slight humming sound as it moves.The time signal it generates pushes the second hand, minute hand, and hour hand to rotate to indicate the time.The error of this watch is small, within 2 seconds per day.This is the second generation.

Compared with mechanical watches, the biggest difference between these two electronic watches is the difference in power, and the vibration system that mainly affects the accuracy of travel time is still mechanical vibration.The third and fourth generation electronic watches are all products of the reform of the oscillation system.

In the 60s, semiconductor integrated circuits appeared, which miniaturized electronic components, and people made quartz electronic watches.This is the third generation, which is dozens of times more accurate than mechanical watches, with an annual error of 60 to 180 seconds.

In the 70s, a liquid crystal display type quartz electronic watch appeared, which is the fourth generation. Its annual error is less than 30 seconds, and the travel time accuracy is even higher.

Quartz clock

A quartz clock is a clock driven by electric current, but it is different from a general electric clock.It has a piece of quartz crystal inside that oscillates due to voltage when current is passed through it.Its oscillation frequency is very stable, and the controlled electronic oscillator makes the travel time very accurate.The best quartz clock is only 1000 second every 1 years. The observatory not only uses it to keep time, but also uses it to measure the change of the earth's rotation speed.

A single crystal of natural quartz is a crystal, which chemists call silica, a compound of silicon dioxide.Crystal can not only be used as ornamental arts and crafts, but more importantly, it is the "key point" of the electronics industry.

In 1880, the French scientists Pierre Curie and Riac Curie brothers discovered the important phenomenon of crystal in physics.They cut the crystal into parallel thin slices, placed between two metal plates, and pressed or elongated the thin slices under the action of external gravity, and a strange phenomenon occurred: the opposite ends of the wafer accumulated. symbol charge.

This characteristic of crystal can make the crystal slice vibrate at high frequency under the action of alternating current to produce ultrasonic oscillation.

During the First World War, Germany used submarines to attack the British and French allied forces suddenly, and the ships sank, making the British navy lose its advantage.Later, the French physicist Lange used the principle that the crystal slices discovered by the Curie brothers can generate ultrasonic waves to create the world's first ultrasonic detector. After returning, the instrument can detect the position of the enemy submarine after receiving the echo wave, thus effectively preventing the sudden attack of the enemy submarine and grasping the initiative of the war.

Crystal resonators made by cutting crystals into single chips are indispensable for the manufacture of quartz electrometers and quartz electronic watches.It is used in artificial satellites, missiles, airplanes, ships, electron microscopes, television, telegraph, telex, radio, etc.

The vibration frequency of a quartz crystal is very stable.The general frequency in quartz electronic watches is 32768 Hz, while the oscillation frequency in high-frequency quartz watches reaches 4194304 Hz, so the travel time is very accurate.

A quartz electronic watch is composed of a quartz crystal oscillator, an oscillating circuit, a frequency division circuit, a shaping amplifier circuit, and a micro-motor, and is integrated on a small silicon chip by means of an integrated circuit.It can output a pulse electrical signal every 1 second.

The vibration frequency is fast, and the travel time is very accurate, with a monthly error of 5 to 15 seconds.Use an amplifying circuit to turn the electrical signal into a strong current to drive the motor. This is the analog quartz watch.

Since the 60s, quartz clocks have been greatly improved, and the error of a day generally does not exceed 50-500 microseconds.

accurate atomic clock
Today, with the rapid development of modern science and technology, atomic energy, aerospace technology and high-energy physics require more precise measurement of time.Some isotopes and various particles disintegrate in a fraction of a billionth of a second.Modern electronic computers need to perform calculations in tens of millions of seconds, hundreds of millions of seconds, or even billionths of a second.

Modern technology requires a more precise international standard time.Because, if there is an error of one second, the sailor navigating with a sextant may have a deviation of 1/4 mile; with a difference of 1‰ second, the spacecraft can fly 10 meters; every second, the computer can calculate 80 Time... Greenwich Mean Time is not accurate enough, but it has been used in the world, and there needs to be a compromise to solve it.

Since the 60s, the International Time Bureau has determined the world's coordinated time, so that the time can not only ensure the uniformity, but also reflect the characteristics of the earth's rotation.The international astronomical community defined the atomic second in 1967 and introduced the atomic time timing system. In 1967, the International Conference on Weights and Measures defined the second as "a second (S) is the duration of 133 cycles of radiation corresponding to the leap between the two hyperfine energy levels of the ground state of the cesium-9192631770 atom".That is to say, the atomic clock uses a certain state of motion inside the cesium atom to measure time.

Atomic clocks are the most accurate clocks in the world.It turns out that the electrons inside the atom will radiate electromagnetic waves when they transition, and its transition frequency is extremely stable.Using this electromagnetic wave to control the electronic oscillator, thereby controlling the travel time of the clock, this is the atomic clock.

According to international regulations, at zero o'clock on New Year's Day in 1958, the atomic time is consistent with the universal time, and the difference with the almanac time is 32.15 seconds.This is called the International Atomic Time, recorded as TAT.

From 1972:0.9 on New Year's Day in [-], it broadcasts the universal time through a dedicated radio station, changing from TAT to coordinated universal time, which is recorded as UTC.The so-called coordinated universal time is the universal time coordinated by the International Atomic Time.It is not only different from the international atomic time, but also different from the universal time of the atom.However, the difference between it and the international atomic time is always a complete number of seconds; the difference between it and TAT is always within [-] seconds.

Atomic time can provide a uniform time interval and a standard second length, which can meet the needs of radio, physics, etc. It is very accurate to use it to measure time periods.The atomic clock calculates the lifespan of the elementary particle "meson" to be about 0.000002 seconds; it takes about 0.005 seconds for a bee to beat its wings once; when the spaceship flies to the moon, it flies 10 kilometers per second; the distance traveled by light in vacuum every 29992458th It is 1 standard meter.

The second length of Coordinated Universal Time is strictly equal to the second length of Atomic Time. When the difference between its time and Universal Time exceeds 0.9 seconds, a leap second is introduced into Universal Time.

In order to coordinate the time measurement work of countries all over the world, the International Time Bureau was established in Paris, France.It maintains the international atomic time scale and provides accurate time data for the timing centers of various countries.

The international time bureau in the computer room of the atomic time comparison system of Shaanxi Astronomical Observatory adjusts twice a year, and transmits standard time signals to all parts of the world through the standard time radio station.An increase of 1 second is called a positive leap second, and a decrease of 1 second is called a negative leap second.In this way, the error caused by Greenwich Mean Time can be adjusted. On New Year's Eve in 1979 and on June 1981, 6, the universal time was increased by 30 "leap second" correction time, and the old man was late both times.

This is the universal time currently used by civilians, and it is also the time used by astronomy, geodesy and satellite tracking.

The world's first atomic clock, the ammonia clock, was made by the US National Bureau of Standards in 1949, which marked a new era of time measurement and time guide.In the following ten years, atomic clock technology has made great progress, and rubidium clocks, cesium clocks, and hydrogen clocks have been made successively.By 1992, atomic clocks were in general use around the world.my country has successively manufactured rubidium atomic clocks and hydrogen atomic clocks, which have entered the world's advanced ranks in terms of timekeeping.

Timekeepers of the universe - pulsars
Astronomers have discovered that a rapidly rotating pulsar in the constellation of Vela, if its rotation rate is used as a timer, it will only have an error of 300 second every 1 million years, which is 10 times more accurate than the current "atomic clock". Therefore, astronomers call it "astronomical clock".

Pulsars are one of the four major discoveries in astronomy in the 60s.It has a fairly stable and short pulse period.It is generally believed that it is a rapidly rotating neutron star or a neutron star that is rapidly expanding and contracting.The density of matter inside a pulsar is unbelievably high.A piece of matter the size of a matchbox weighs 20 billion tons.Because it has such a high density of matter, coupled with the constant interaction of its internal extremely strong electric and magnetic fields, it can spin rapidly without disintegrating.

A pulsar is a very peculiar star. It continuously emits radio pulses very regularly, and always sends out radio signals very regularly, just like a pulse beats.The pulses of different pulsars have long or short periods, the long ones are a few seconds, and the short ones are only a few tenths of a second, but the pulse period is very precise.

Why do the radio waves emitted by pulsars have precise periods?It is generally believed that neutron starlight has a structure called a "lighthouse" (area with relatively concentrated radiation), which continuously emits strong radio waves. Every time the neutron star rotates once, the "lighthouse" faces us once, and people receive it once. radio waves.

Who discovered pulsars? In August 1967, when Professor Ish and graduate student Bell of the University of Cambridge were observing the radio source of a huge antenna array with an area of ​​more than 8 square meters, they accidentally recorded a pulse image.This is very puzzling, the observer said humorously:
It's a "signal from the little green man."After a period of repeated observations, it was determined that it was a special new type of celestial body - a pulsar.

This pulsar is called CP1919 (CP is the abbreviation of the pulsar discovered by Cambridge University, and 1919 is the coordinate number of this star), and its pulse period is 7.337301344 seconds, which is accurate to 9 decimal places!In other words, its accuracy is as high as 20/[-]th of a second![-] times higher than cesium atomic clock.

Later, the United States, Australia and other countries successively observed many pulsars, and up to now there are more than 330 pulsars.

Although the name of a pulsar is called a star, in fact, it cannot be seen even with the largest optical telescope, and its pulse signal can usually only be received with a radio telescope.

General pulsars have only been found to have radio pulses, while some pulsars have also been found to have optical pulses.The observation records of the superstar explosion in 1054 in ancient my country provided very valuable information for the formation and evolution of pulsars.

In the early morning of July 1054 in the Song Dynasty, astronomers at the Qin Tianjian saw an astonishing celestial phenomenon.An object brighter than Venus appeared in the east and was visible during the day for 7 consecutive days.Then the brightness gradually weakened, and it took more than 23 days before it was completely invisible.The history of the Song Dynasty recorded the whole story of this guest star and called it "the guest star of Tianguan in 60".After it erupted, it left a cloud-like remnant, photographed with a telescope, shaped like a crab, so it is called the Crab Nebula.It is also now constantly emitting radio waves in all directions.

Scientists believe that the "astronomical clock" is the most accurate clock in the universe. In future space exploration, pulsars can replace the "atomic clock" as a timer for interstellar flights.

A 'clock' to determine ancient ages

Since 1979, the Institute of Archeology of the Xinjiang Academy of Social Sciences has sent people to the Lop Nur desert area three times to conduct investigations, initially uncovering the mystery of the Loulan Kingdom.A brown young female corpse was found on the north bank of the Peacock River. Her hair was slightly curly and her eyes were closed, just like a sleeping ancient Loulan girl.

This young Loulan female corpse has aroused the interest of some archaeologists, but how to determine how many years since she died?If there are funerary objects in the tomb, it can be identified which age it belongs to.Scientists speculate that the Loulan female corpse is more than 2000 years old.

Some well-preserved ancient lotus seeds were unearthed in the mud layer of Migawa, Chiba Prefecture, Japan.American scientist Li Bi was invited to identify how old the ancient lotus seeds lived. He determined that these seeds were 3000 years old.The ancient lotus seeds bloomed and bear fruit after being cultivated.

Scientists have found one of nature's "clocks" -- radiocarbon-14.This kind of clock does not need to be wound by people, and it is not affected by external temperature, pressure, etc., and it always moves correctly and continuously.It can be used to determine how many years old some substances are.

At the beginning of the 20th century, when Austrian physicist Hess placed an electroscope in a balloon basket to measure the degree of air conductivity, he discovered a ray from outside the sky, which attracted the attention of many scientists. In 1930, American scientist Libby discovered that when these rays pass through the earth's atmosphere, many high-energy neutrons will be produced, and these neutrons will hit the nitrogen atoms in the air like rain particles, turning the nitrogen atoms into a kind of A new carbon atom that has 6 protons and 8 neutrons.

Radioactive carbon 14 is an unstable isotope, it will continue to emit radiation and reduce, and at the same time continue to produce in the atmosphere to keep the carbon content in balance.

All organisms on the earth, when they are alive, always absorb the carbon dioxide in the atmosphere continuously, and must also absorb the mixed carbon 14.Only when animals and plants die, they stop material exchange with the outside world, and the supply of carbon 14 also stops.Since then, the content of carbon-14 has gradually decreased due to the continuous emission of radiation.About every 5568 years, the carbon-14 content decreases by half.This is called the "half-life" of the radioisotope.In this way, if you want to know the growth age of the ancient corpse or ancient lotus seeds, you only need to measure the content of carbon 14 in the ancient corpse or ancient lotus seeds, and you can calculate it.

Archaeologists use carbon clocks to date artifacts.For example, a boat-shaped container was unearthed in an Egyptian tomb. Archaeologists removed a piece of wood from the container, and after carbon clock determination, it was 3620 years ago.Chinese archaeologists used carbon clocks to determine that Banpo Village in Xi'an is a Neolithic site, about 6000 years ago.

However, using carbon clocks to judge the ancient geological age, because its travel time is too short, only tens of thousands of years, and the lack of carbon in the rocks, it seems powerless.Geology chooses the common radioactive element potassium 40 in rocks as the "potassium clock".

Potassium 40 will become Argon 40 after emitting radiation.Therefore, as long as the content of potassium 40 and argon 40 in the rock is determined, and then calculated, the age of the mineral or rock can be inferred.Because radioactive potassium 40 has a longer half-life, it can be used to judge the age of fossils billions of years from today. Using potassium clocks, people have determined that the rocks on the top of Mount Everest were formed 4.5 million years ago.The oldest rocks on Earth are about 40 billion years old.Rocks brought back by astronauts from the moon have been determined to be 45 billion years old.

greek water clock
About 1000 years ago, the Greeks made more sophisticated water clocks.Its structure is like this: there is a small hole on the upper side of the water storage jug, and excess water can overflow from this small hole, so that a fixed level can be maintained and a constant pressure can be maintained.Water flows out from the small hole in the lower part of the water storage pot and pours into the water receiving pot.There is a floating boat in the kettle.The floating boat is equipped with "Guardian Bell God"

--shaft.When the water in the jug reaches a certain height, the water is siphoned into the rotating balance wheel (which turns by its own weight), which turns a series of gears, so that according to the length of day and night, the drum-shaped cylinder used for timing is brought to the new location.As the height of the water surface of the kettle changes, the "Guardian of the Bell" will point out the time on the cylinder's engraved line.These engraved lines are of unequal length, and some are inclined to indicate the time of day in winter.

Astronomical Observatory

(End of this chapter)