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CHILDREN'S SCIENTIFIC LABORATORY
Quartz watch. Children's Science Lab
Directory / Children's Science Lab What time is it now? We are accustomed to finding the answer to this question by looking at the clock; manual, pocket, desktop, wall, street, tower. You can check the time by telephone and radio. Broadcasting stations in the Soviet Union transmit accurate time signals four times a day. How did our ancestors keep track of time? Five thousand years ago, people used the Sun Clock for this - an ordinary pole, installed vertically and casting a shadow of different lengths and directions at different times of the day. Later, time was measured using water and hourglasses. The accuracy of these primitive instruments was, of course, very approximate. By the XNUMXth century AD, the invention of mechanical - tower clocks dates back, and five hundred years later, the first spring clocks appeared. However, they did not differ in great accuracy, since the speed controller - the balancer - fluctuated unevenly with them. This shortcoming was eliminated when the property of a freely suspended pendulum was discovered to keep the period of its oscillations constant. By connecting the pendulum with the clockwork, we got a device for measuring time with sufficient accuracy. Continuous constructive improvement of pendulum clocks has made them a reliable time meter. Demanding consumers But science and technology do not stand still. Simultaneously with their development, the requirements for the accuracy of time determination increased. Accurate to one second time ceased to satisfy many of its "consumers". They wanted to know time to the nearest hundredths, thousandths, even ten-thousandths of a second. These were not only astronomers studying the movement of heavenly bodies. The navigators of ships and aircraft required the most accurate time for correct orientation at sea and in the air, topographers and surveyors surveying the area. To establish where they are on the globe, they had to determine the geographical latitude - the distance from the equator - and longitude - the angle between the plane of the meridian of a given place and the plane of the zero meridian. To correctly determine longitude, it is necessary to know the local time and the time on the zero meridian with the utmost accuracy, since longitude is calculated from the difference between these two values. Let it be determined by the stars that it is 23 hours 30 minutes at a given moment in a given place. The clock, set to the time of the zero meridian and checked by radio, shows 21 hours 30 minutes. The difference is two hours. It is known that the Earth per day makes one revolution from west to east around its axis, i.e., it rotates by 360 °, and in one hour - by 360:24 = 15 °. In two hours it will turn 30°. Therefore, the observer is at 30° east longitude. The exact time must also be known to geologists and gravimetrists who study changes in gravity at various points on the earth's surface, which is of great importance for the exploration of minerals. Heavenly clock How is the exact time that plays such an important role in people's lives determined? What ultra-precise clocks do scientists check their clocks against? These wonderful watches are created by nature itself. Their dial is the night sky, and the numbers for hours, minutes and seconds are the stars. With strict constancy they follow their eternal path in the firmament. Invariably, at precisely the moment set by astronomers, each star reaches its highest position and crosses the celestial meridian. It is enough to catch this moment to find out the perfect time. To cope with this task, astronomers are helped by the "hand" of the celestial clock - a special astronomical tube called a transit instrument. Rotating simultaneously with the Earth, the transit instrument is always directed along the meridian, indicated in the field of view of the instrument by a thin vertical thread. By noting the passage of a star through this filament, the astronomer calculates how much he should correct his watch. Every night, astronomers from all observatories in the world sit down at the passage instruments. But the weather is not always conducive to observations. They need a clear sky, and, for example, in Moscow there are only about 90 cloudless nights a year, in sunny Tashkent - about 250. Often the sky is covered with clouds for a whole month in a row, and sometimes even longer. It was necessary to find a way to set the exact time during these forced breaks from one astronomical observation to another. So there was a problem of "storage" of time. The resolution of this complex problem was facilitated by the creation of high-precision astronomical clocks. Two pendulums The main and most important part of the astronomical clock is the pendulum. It's clear. After all, the main advantage of the watch lies in the uniformity and constancy of their course. But the clock can go evenly only if the length of the pendulum always remains strictly constant and the amplitude of its swings is unchanged. What can influence these values? First of all, changes in temperature and air pressure. It follows that the pendulum must be made of a material that is least affected by changes in temperature. Invar turned out to be such a material - an alloy consisting of 36% nickel and 64% steel and having a coefficient of linear expansion 10-12 times less than steel. A pendulum was made from Invar. The designers of astronomical clocks took other precautions as well. They placed the clock in the cellar, where the temperature changes little, and enclosed it in a hermetically sealed copper cylinder with a glass cap. Air is almost completely pumped out of the cylinder, and the atmospheric pressure in it is constantly maintained within the range of 20-25 mm of mercury. The clock was installed on a special foundation isolated from the building. Therefore, they are not very sensitive to the vibrations of the building in which they are located. They also took care to free the pendulum from any, even the slightest, mechanical load. This is the main idea of high-precision astronomical clocks. The free-swinging pendulum, which we have described, is not connected with any transmission and time-indicating mechanisms. It's called a "free" pendulum. His mission is limited. It only measures time, and all the "black" - mechanical work is assigned to another, auxiliary pendulum. The free pendulum receives swinging impulses every 30 seconds. They are sent to him by wires by an auxiliary pendulum. With the help of special electrical devices, the free pendulum, as it were, commands the auxiliary pendulum, forcing it to oscillate strictly synchronously with itself. The auxiliary pendulum operates a transmission mechanism that moves the hands on the dial. This second clock, connected by electrical wires to the first, can be installed anywhere, at any distance from the main pendulum - the true keeper of time. All astronomical observatories and metrological institutes of the world now use clocks with two pendulums in their work. The accuracy of such clocks is extremely high: their course, being adjusted, varies from day to day by no more than 0,003 seconds. Such accuracy seems fabulous, however, it is not sufficient for modern science, because an error of even a few thousandths of a second prevents the study of certain phenomena that are of interest to astronomers, metrologists and geophysicists. Miraculous property of crystals Where to look for a way out? The mechanics seemed to have exhausted all their possibilities and reached the limit: further improvement of the pendulum clock seemed unthinkable. And then electricians and radio engineers took up the design of astronomical clocks. The pendulum has outlived its time, they argued. Even placed in ideal conditions, the pendulum is not able to meet the increased demands of scientists. This means that it is necessary to replace it with another regulator that provides oscillations of a constant frequency. In search of such a regulator, they remembered quartz. Quartz crystal and its axes
In 1880, a remarkable property of some crystals was discovered, most pronounced in quartz. Quartz is usually found in the form of hexahedral crystals with pointed pyramidal ends (Fig. 1a). The zz line represents the optical axis of the crystal. If the crystal is cut across, perpendicular to the optical axis, then a hexagon is obtained, all angles of which are equal to 120° (Fig. 1b). The lines xx, x1x1 X2X2 passing through the bisectors of these angles denote the electrical axes, the lines yy, Y1Y1, Y2Y2 - the mechanical axes of the crystal. It turned out that if a plate is cut from a quartz crystal, the surfaces of which are perpendicular to one of its electrical axes, then when the plate is mechanically compressed or stretched, electric charges arise on its surfaces. This phenomenon is called the direct piezoelectric effect (the ancient Greek word "piezo" means: I press, squeeze.). The reverse piezoelectric effect is expressed in the deformation of a quartz plate placed in an electric field. Shortwave radio amateurs are well aware of this property of quartz. They know that the quartz plate has the ability to keep the oscillator frequency constant. Quartz stabilizers are widely used in radio stations. It was this stabilizing ability of quartz that the creators of the new time keeper decided to use. Quartz watch Designers of quartz watches cut out a rectangular bar from a crystal with a section of 7x7 mm and a length of about 60 mm. On two opposite surfaces of the bar, they applied the thinnest layer of gold. The result was a capacitor, the dielectric of which is a bar, and the plates are two layers of metal. The purpose of this device in quartz clocks is the same as that of the pendulum in ordinary clocks: it is a regulator. And a regulator that you can completely rely on. Quartz crystal in the frequency-setting circuit of a triode
Then the quartz was included in the circuit of the lamp generator. The crystal was placed in a grid circuit - generator lamp cathode - triode (Fig. 2). In parallel, a large resistance was installed. An oscillating circuit consisting of an inductor and a capacitor was included in the anode circuit of the circuit. This is necessary so that, due to the connection through the capacitance of the anode - grid of the lamp, conditions are created to maintain undamped oscillations. The circuit was tuned so that its natural frequency was higher than the oscillation frequency of the quartz bar. This is in general terms the device of a quartz oscillator - the main part of a quartz watch. Their accuracy is directly dependent on the stability of the oscillator frequency. The constancy of natural oscillations of quartz is very high. It is not affected by either changes in the force of gravity or seismic vibrations of the earth's crust. However, it is sensitive to fluctuations in temperature and atmospheric pressure. In order to keep the quartz temperature constant, the designers took special measures. They placed the crystal oscillator in a thermostat with multi-layered walls, inside which a constant temperature is maintained with an accuracy of one hundredth of a degree. This temperature constancy is achieved by electrical heating of the thermostat controlled by a mercury contact thermometer. This ensures that the frequency is stored with an accuracy of about 1*10-8. The quartz itself was enclosed in a hermetic vessel in which a vacuum was created. Quartz oscillator with frequency dividers
The designers machined a block of such a shape and size from a quartz crystal that its natural frequency was 100 kHz. But the current of this frequency is not suitable for the rotation of the motor that sets the clockwork in motion. I had to create a number of intermediate devices shown in the block diagram (Fig. 3). Here, electronics helped the designers a lot. A number of electronic generator circuits have the ability to synchronize with the frequency of another generator if it is a multiple of the number of times higher or lower than the natural frequency of the synchronized generator, or close enough to such a multiple value. The designers of quartz watches took advantage of the ability of circuits such as the multivibrator or blocking oscillator to synchronize to higher frequencies than their own. Such a synchronized, higher frequency oscillator is commonly referred to as a frequency divider. The highest frequency of current that can drive a synchronous motor is about 1000 Hz. However, a frequency divider with a division ratio of 1:100 is very unstable. Therefore, to obtain a frequency of 1000 Hz, synchronous with a quartz frequency of 100 kHz, it was necessary to install a number of dividers with ratios of 1:4 and 1:5, synchronizing each other in series. The generators used as frequency dividers have a large number of harmonics. It was necessary to prevent the penetration of harmful high-frequency oscillations into the crystal oscillator circuit, where they could cause deterioration in stability. To prevent this from happening, a buffer amplifier was connected between the quartz oscillator and the first frequency divider, which operates without grid currents. This mode helps to reduce the load of the crystal oscillator and increase the stability of its operation. In frequency divider circuits, low-power lamps are usually used. The current they provide is too weak to turn the synchronous motor driving the seconds contact clockwork. Therefore, after the frequency divider (giving a current with a frequency of 1000 Hz), an amplifier was turned on, giving several watts of power to the motor windings. In terms of stability, quartz clocks are superior to all existing pendulum clocks. The average daily fluctuation of their course is two ten-thousandths of a second. The creation of ultra-precise clocks is an outstanding achievement of modern science. Many scientific institutions have already acquired quartz clocks. In Moscow, at the Central Research Institute of Geodesy, Aerial Photography and Cartography, the first domestic quartz clock built by PS Popov tirelessly measures seconds. The Institute of Radio Measurements, the Sternberg Astronomical Institute and other institutes and observatories have quartz clocks. Enthusiasts of the new way of measuring time claim that quartz clocks will soon completely replace pendulum clocks and become the only keepers of time. There are also skeptics who dispute such claims. Without denying the obvious advantages of quartz watches, they also point out their disadvantages. We have already talked about the advantages of quartz watches; this is their unsurpassed accuracy and constancy of course, independence from almost all external factors. What are their shortcomings? Astronomers demand that the clock they use to measure time can run non-stop for two, three years or more. Do quartz watches meet this requirement? Not quite. Recall that they are powered by current from the electrical network. The station will stop supplying current, and the clock will stop. But this will not happen if the watch is powered not from the network, but from batteries. - That's right, - skeptics agree. - And what about the aging of quartz, with the wear of radio tubes? Indeed, quartz ages over time, and the frequency of its oscillations changes. You can not guarantee that any lamp will not suddenly fail. However, quartz enthusiasts are not afraid of such an accident. They install in their laboratories not one clock, but three, working synchronously. It doesn't matter if one of them stops. Until they are repaired, the other two will keep time. The dispute continues, but meanwhile, dozens of quartz watches regularly serve science. Today, their accuracy satisfies scientists conducting the most delicate research. And what will happen tomorrow? Will it be possible to find a new standard of time, even more accurate? Perhaps the basis of such a standard will be molecules, or rather, the frequency of their vibrations. Soviet scientists are already working in this direction. Author: A. Brodsky
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