12 June 2009

Electric clock

The invention: Electrically powered time-keeping device with a quartz resonator that has led to the development of extremely accurate, relatively inexpensive electric clocks that are used in computers and microprocessors. The person behind the invention: Warren Alvin Marrison (1896-1980), an American scientist From Complex Mechanisms to Quartz Crystals William Alvin Marrison’s fabrication of the electric clock began a new era in time-keeping. Electric clocks are more accurate and more reliable than mechanical clocks, since they have fewer moving parts and are less likely to malfunction. An electric clock is a device that generates a string of electric pulses. The most frequently used electric clocks are called “free running” and “periodic,” which means that they generate a continuous sequence of electric pulses that are equally spaced. There are various kinds of electronic “oscillators” (materials that vibrate) that can be used to manufacture electric clocks. The material most commonly used as an oscillator in electric clocks is crystalline quartz. Because quartz (silicon dioxide) is a completely oxidized compound (which means that it does not deteriorate readily) and is virtually insoluble in water, it is chemically stable and resists chemical processes that would break down other materials. Quartz is a “piezoelectric” material, which means that it is capable of generating electricity when it is subjected to pressure or stress of some kind. In addition, quartz has the advantage of generating electricity at a very stable frequency, with little variation. For these reasons, quartz is an ideal material to use as an oscillator.The Quartz Clock Aquartz clock is an electric clock that makes use of the piezoelectric properties of a quartz crystal. When a quartz crystal vibrates, a difference of electric potential is produced between two of its faces. The crystal has a natural frequency (rate) of vibration that is determined by its size and shape. If the crystal is placed in an oscillating electric circuit that has a frequency that is nearly the same as that of the crystal, it will vibrate at its natural frequency and will cause the frequency of the entire circuit to match its own frequency. Piezoelectricity is electricity, or “electric polarity,” that is caused by the application of mechanical pressure on a “dielectric” material (one that does not conduct electricity), such as a quartz crystal. The process also works in reverse; if an electric charge is applied to the dielectric material, the material will experience a mechanical distortion. This reciprocal relationship is called “the piezoelectric effect.” The phenomenon of electricity being generated by the application of mechanical pressure is called the direct piezoelectric effect, and the phenomenon of mechanical stress being produced as a result of the application of electricity is called the converse piezoelectric effect. When a quartz crystal is used to create an oscillator, the natural frequency of the crystal can be used to produce other frequencies that can power clocks. The natural frequency of a quartz crystal is nearly constant if precautions are taken when it is cut and polished and if it is maintained at a nearly constant temperature and pressure. After a quartz crystal has been used for some time, its frequency usually varies slowly as a result of physical changes. If allowances are made for such changes, quartz-crystal clocks such as those used in laboratories can be manufactured that will accumulate errors of only a few thousandths of a second per month. The quartz crystals that are typically used in watches, however, may accumulate errors of tens of seconds per year. There are other materials that can be used to manufacture accurate electric clocks. For example, clocks that use the element rubidium typically would accumulate errors no larger than a few tenthousandths of a second per year, and those that use the element cesium would experience errors of only a few millionths of a second per year. Quartz is much less expensive than rarer materials such as rubidium and cesium, and it is easy to use in such common applications as computers. Thus, despite their relative inaccuracy, electric quartz clocks are extremely useful and popular, particularly for applications that require accurate timekeeping over a relatively short period of time. In such applications, quartz clocks may be adjusted periodically to correct for accumulated errors. Impact The electric quartz clock has contributed significantly to the development of computers and microprocessors. The computer’s control unit controls and synchronizes all data transfers and transformations in the computer system and is the key subsystem in the computer itself. Every action that the computer performs is implemented by the control unit. The computer’s control unit uses inputs from a quartz clock to derive timing and control signals that regulate the actions in the system that are associated with each computer instruction. The control unit also accepts, as input, control signals generated by other devices in the computer system. The other primary impact of the quartz clock is in making the construction of multiphase clocks a simple task. A multiphase clock is a clock that has several outputs that oscillate at the same frequency. These outputs may generate electric waveforms of different shapes or of the same shape, which makes them useful for various applications. It is common for a computer to incorporate a single-phase quartz clock that is used to generate a two-phase clock.


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