Encyclopedia of popular science knowledge: "Time Knowledge"

Chapter 4 All kinds of clocks (4)
When the doctor checks the hearing, the thing that is hit with a small hammer is the tuning fork.The doctor put the tuning fork that had been struck with a small hammer near the patient's ear. Within a period of time, the patient heard a continuous "humming..." sound, which indicated that the tuning fork was vibrating.If the tuning fork is made of a metal material that is less affected by temperature, this vibration is very stable.

Of course, the tuning fork used to make the "tuning fork electronic watch" is much smaller than the tuning fork held in the doctor's hand. How does the tuning fork electronic watch work?After the battery supplies power to the oscillator, the oscillator oscillates. The magnetic field of the inductance coil interacts with the magnetic steel fixed on the top of the two arms of the tuning fork to drive the tuning fork to vibrate, and its vibration frequency in turn controls the oscillation frequency of the oscillator. , so that the oscillation frequency of the entire oscillation system is mainly determined by the vibration frequency of the tuning fork, which is the so-called frequency stabilization effect.One arm of the tuning fork protrudes a claw, and when the tuning fork vibrates, it pushes the counting wheel, which turns the entire gear system and drives the pointer to move.

In the tuning fork electronic watch, the tail of the traditional hairspring and balance wheel system has been cut off, and it is developing towards a higher level, and the accuracy of travel time is correspondingly improved, and the daily error is within 5 seconds.

The third generation of electronic watches are analog quartz watches.

The travel time accuracy of the tuning fork electronic watch has been improved, but it still cannot meet people's requirements for precise time. People began to use a more ideal frequency stabilizing component-quartz crystal.Quartz crystal has very stable physical and chemical properties, and its frequency stabilization effect is excellent.

The third-generation electronic watches are mainly composed of micro batteries, quartz crystals, integrated circuits, micro motors and gears, and indicating systems.

As a frequency stabilizing element in the oscillator circuit, the quartz crystal forms an oscillation together with the integrated circuit after the power is turned on, generating a very stable 3Z hertz signal, which is also transformed into an oscillation per second (1 Hz) by the integrated circuit. The signal is amplified to a sufficient strength to drive the micro motor to drive the gear and the pointer to rotate.

The fourth generation of electronic watches still use quartz crystals as frequency stabilizing components, but its mechanical structure has been reduced to the minimum, and even the traditional gears and hands are gone.Instead of gears are integrated circuits, and instead of hands are light-emitting diodes or other display elements.People call the fourth generation of electronic watches "digital display quartz watches".

In the history of human watchmaking, due to the use of quartz crystals as frequency stabilization components and the use of integrated circuits, major changes have taken place in the manufacture of watches.Quartz watches are currently the most accurate watches in the world, with an error of only 0.1 seconds per day, less than half a minute a year.At the same time, it can run automatically, is easy to use, and has a novel, beautiful and generous form, which makes its watch partners pale in comparison.

At present, electronic watches are developing toward high-precision, thin, small, and multi-functional aspects.Some electronic watches have as many as 20 functions. In addition to displaying hours, minutes, seconds, day, week, and month, they can also display the world time, and some can also be used as alarm watches and stopwatches.

In terms of energy, people have now developed photovoltaic cells and solar cells to replace the original micro-batteries, and are studying the way of using human body temperature as a new energy source for electronic watches. This is a more convenient and practical method.

3 years to one second clock - atomic clock
Today, we not only have crystal clocks with a difference of 300 second in 1 years, but also more advanced atomic clocks.For example, the cesium atomic clock looks like a square and not too big cabinet, which is covered with various switches, knobs, red and green indicator lights... If you don't see the prominent characters on it "cesium beam tube atomic clock" ", I would never believe that this is a clock, because it is completely different from ordinary clocks.

Cs-beam tube atomic clocks and other types of atomic clocks are all timed by the frequency of "original transition". This frequency is very high and extremely stable, so its timing accuracy is also very high, which can reach 3 years with a difference of 1 Second!Although the crystal clock we mentioned above is already very accurate, it has aging drift and other phenomena, so it cannot be used as a first-class time-frequency standard, while atomic clocks are currently recognized as a first-class time-frequency standard in the world.

The atomic clock is awesome, it is only 3 second off in 1 years, how accurate it is!
With the development of human cognition towards the microcosm, and after revealing the secrets of atoms, a more accurate method for human timing is provided, because the stability of the microcosm far exceeds that of the macrocosm.

There are no identical individuals in the macroscopic world. A pair of twin brothers look very similar, but if you observe carefully, you can find their differences; TVs produced by the same manufacturer with the same components have exactly the same appearance, but the quality But each has its pros and cons.

In the microcosm, on the contrary, there are many identical things, and we cannot distinguish one electron from another, nor can we distinguish one atom of the same element from another.This is not because our measuring instruments are too clumsy, but they are indeed identical, and in principle it is impossible to tell them apart.Even under the high temperature and high pressure deep in the earth, their properties cannot be changed.

All timing methods and timing tools are based on regular changes in objects.For example, the earth rotates once a year around the sun, the earth rotates once a day, the balance wheel of an ordinary watch swings 1 ​​times per second, and the crystal clock oscillates 1 times per second, while the atomic clock uses the "vibration" of atoms to time. It vibrates billions of times per second.The improvement of timing frequency itself means the improvement of timing accuracy.

Since the advent of the atomic clock in the Indian era, it has developed into a "family".There are the original ammonia molecular clocks, rubidium atomic clocks, cesium atomic clocks, and hydrogen atomic clocks... Each clock has a different type.

There are many different types of atomic clocks, but they all work on the same principle.They all take advantage of the feature that the periodicity of atomic transitions is stable.

300 years to one second clock - crystal clock
With a high stability crystal oscillator, as in an electronic watch, it can only be said that with an advanced "electronic pendulum", it does not constitute a complete clock by itself.

The oscillation frequency of a high-stability crystal oscillator is generally 2-[-] times per second, and each oscillation cycle is only a few millionths of a second, that is, a few tenths of a microsecond. Such a small time scale is very important for radio technology and time. For the measurement of frequency, it is very accurate and convenient, but for our traditional timing concepts of hours, minutes and seconds, such a time scale is too small.

If we can transform the standard signal of the crystal oscillator so that it generates a signal that oscillates once per second, once per minute, and once per hour... the lower the frequency, the longer the period, which is similar to the one we use every day. The clocks correspond one by one.These low-frequency signals are then displayed in digital or mechanical form, which constitutes a standard clock whose stability is determined by a crystal oscillator—a crystal clock.

The development of modern electronics makes it easy for people to realize the above assumptions.Crystal clocks have already been manufactured. As early as 1927, Bell Telephone Laboratories in the United States, Marison, was the first to develop a crystal clock to measure time; in 1933, the Tokyo Observatory was first equipped with a crystal clock to keep accurate time.Up to now, crystal clocks have been more widely used in various fields, whether in the standard room of the Bureau of Metrology, on the tall buildings in the square, or in the hall of sports competitions, we can find crystal clocks the "trace".While their timekeeping accuracy requirements vary, their basic operating principles are the same.

A crystal clock is generally composed of the following parts.The high-stable crystal oscillator sends the 5 MHz (or 2.5 MHz) standard signal to the first frequency divider, and divides the frequency 5×106 times to obtain a signal of one cycle per second, that is, the "second" signal; Frequency divider, frequency division and printing times, you can get a signal of one cycle every 60 seconds, that is, a "minute" signal; and then pass through the third frequency divider, continue frequency division and printing times, you can get a signal of one cycle every 3600 seconds, that is, "hour" Signal.Send the separated second, minute and hour signals to the decoding display circuit, and then the ×× hour, ×× minute, and ×× second can be displayed in digital form, which is very convenient to read like a quartz watch.There are various modern digital display methods, such as digital tube display, light-emitting diode display, liquid crystal display and plasma display.The displayed colors are red, orange, green... The numbers flicker and jump, which is very intuitive and beautiful.

This alone is not enough, we can imagine that if the oscillator continuously outputs standard signals, and the time accumulates minute by minute, there will be a situation of "25 hours × × minutes × × seconds", and no matter how long the time is, it will appear 35 hours, 48 ​​hours... This is different from our actual application.Therefore, a set of adjustment circuit must be added. When the time is accumulated for 34 hours, the entire system will be completely restored to zero, and the timing will restart. This is the function of the "reset circuit".Just like our commonly used clocks and watches, the maximum value indicated is 12 o'clock. After 12 o'clock, the reading of the pointer starts again.

If we use the method of frequency conversion to drive a mechanical device—a synchronous clock with the obtained second signal, a second signal makes the second hand of the synchronous clock jump once, and drives the minute hand and hour hand, which constitutes a crystal clock with mechanical indication. This is more similar to the clocks we use every day.

Speaking of this, it needs to be pointed out that during the signal conversion process, the stability of the high-stable crystal oscillator has not been changed, and the stability of the obtained hour, minute, and second signals can still be maintained at the order of 10-10.In this sense, the high-stability crystal oscillator is equivalent to the "mechanical pendulum" in ordinary clocks and watches. It is a key component for frequency stabilization, so some people call it a "crystal pendulum", which makes sense.

如果1台晶体钟的稳定度是1×10-10，那么它相当于多少年差1秒呢？相当于317年差1秒，通常我们就说成3印年差1秒。这样的钟多准啊！如果我们每一代人按30年来计算，那么3印年就整整是10代人的时间！这就是说，我们上溯10辈的先祖对准的钟表，走到现在，只不过才差上1秒钟！

Measurements over long intervals - radioactive "clocks"

In April 1973, during the construction of the majestic new building of the Beijing Hotel, two ancient trees were found at a depth of about 4 meters underground. One was slightly decayed and soft, and the other was carbonized and hard.According to the determination by the Institute of Archeology of the Chinese Academy of Sciences, these two ancient trees are 13 years old... The ancient trees are buried quietly in the ground, and it is impossible to find records about two ordinary trees in historical documents.Millions of years ago, it was even more impossible for someone to place a clock beside them, and, so far, none of the ordinary clocks, crystal clocks and atomic clocks we have invented can work for millions of years.So, how did scientists determine the age of these two ancient trees?
As you know, in our life, production and scientific experiments, we not only need to measure extremely short time intervals, as mentioned earlier, some time intervals are as short as one billionth of a second; we also need to measure time intervals as long as Time intervals of thousands, hundreds, thousands, and billion years.For example, archaeologists want to measure the age of historical relics and even the origin of human beings.Another example is that geologists need to study the formation age of various strata, and even when the earth itself was formed; astronomers need to study how old the sun and other stars are... These are all important topics for scientists.The problem of determining the age of these two ancient trees is just one of many such problems.

What method do scientists use to measure such a long time interval?All modern scientific fields are intertwined, interpenetrated, and mutually promoted. It is not archaeologists, geologists, or astronomers who first solve these problems, but radiochemists.

Radiation chemists have developed a unique clock.The clock required neither regular winding nor frequent maintenance, yet it could measure extremely long intervals of time.This kind of clock uses radioactive principle to measure time, we call it "radioactive clock".

(End of this chapter)