A Quartz Watch - why is
it so accurate?
The time-keeping of clocks and later watches was regulated by mechanical means until electrical methods were developed. The mechanical regulation relied on devices such as a weight driven swinging pendulum in clocks or an oscillating balance wheel in a spring driven watch.
The accuracy of a modern mechanical watch is mainly dependent on how well the mechanical oscillations of the balance wheel can be regulated. One of the mechanical problems that has to be overcome to produce a good watch is to minimise the friction between the moving parts. This is accomplished by mounting the spindles of the cogs and balance wheel in jewels ... which is why the number of jewels was often displayed on the watch as it was an indication of it's quality. Coincidentally, this month's Image of the Month is a beautiful close up of a mechanical watch which shows the jewelled bearing of the balance wheel.
Although a variety of electrically driven or electronic clocks and watches were developed in this century, it was the widespread use of the quartz crystal that brought cheap and very accurate watches to you and me. It was found that quartz crystals have the ability to vibrate at very high frequency when they form part of an electrical circuit. The frequency was found to be very stable and this property could be used for very accurate time-keeping in a clock or watch with very few moving parts.
A quartz watch is therefore relatively simple. A battery and ancillary circuitry maintains the oscillation of the quartz crystal. The oscillations, which typically occur 100 000 times a second, are accurately scaled down by electrical and mechanical means to drive either an analogue or digital display of the time.
Atomic clocks which are used as fundamental time standards, are even more accurate as they utilise the transitions that occur between certain states of an atom or molecule, which are not dependent on factors such as temperature. These transitions are associated with sharply defined frequencies very much higher than those of quartz crystals.
Further reading: Look up keywords like clock, watch, time-keeping and quartz in the large encyclopaedias such as Encyclopaedia Britannica. Also try multimedia encyclopaedias. Encarta, for example, has a useful overview under 'Clocks and Watches'.
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Video Camera CCD chip - how the image is formed
The charge coupled device image detector (CCD) was developed in 1970 by Boyle and Smith at Bell Laboratories. The CCD chip in a video camera is a two dimensional array of pixels (picture elements) on a silicon chip. In my video camera there are 681x582 pixels in a chip area of 6.4x4.8mm which suggests each pixel is about 10 microns across. The optical lens of the video camera focuses the image on the chip mounted behind the lens.
Each pixel is a tiny capacitor sensitive to light, and photons hitting each pixel build up a charge directly proportional to the amount of light falling on it. The pixel array therefore transforms the optical image shining on the chip into an electronic image in the form of a 2D charge array. The chip is designed so that the charge on each pixel can be moved in sequence to a point where they can be measured. This allows the electronic image built up on the chip to be processed as a video signal by the camera's electronics. This video signal is sent to a video tape recorder (present in the camera for a camcorder) and a small image is also sent to the viewfinder.
The CCD chip described above would only create a black and white image, so how is the colour information detected and processed? In a domestic colour video camera there are alternating groups of pixel elements with red, green and blue filters to record the colour information. The three distinct images for each primary colour are processed separately and combined to create a colour image. Single chip colour CCD cameras typically have a third of the resolution of a black and white one. In top of the range video cameras and those used by TV stations, the camera will use a three chip system (one for each primary colour) to ensure high resolution broadcast quality images.
'CCD Astronomy, Construction and Use of an Astronomical CCD
Camera' by C Buil, Pub. Willmann-Bell, Richmond, US, 1991. ISBN
Encarta '94 has no description of a CCD chip, and Encyclopaedia Britannica (1993) is also sketchy.
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Disclaimer: all the information in this series is given in good faith. However, no responsibility is accepted for damage to property or injury to persons as a result of readers investigating the subjects described. It is up to the reader to judge whether the subjects can be safely viewed in their own home. Younger readers should consult their parents or teacher as appropriate before examining either their own or somebody else's property. Return to top of page