QUARTZ CLOCK

INTRODUCTION

A quartz clock is a clock that uses an electronic oscillator that is regulated by a quartz crystal to keep time. This crystal oscillator creates a signal with very precise frequency, so that quartz clocks are at least an order of magnitude more accurate than mechanical clocks. The first quartz clock was built in 1927 by Warren Marrison and J.W. Horton at Bell Telephone Laboratories.

EXPLANATION

Quartz is a chemical compound called silicon dioxide. Many materials can be formed into plates that will resonate but quartz is the most preferred material for time keeping because of two special qualities that it has: its piezoelectric quality and its stability in every time place and condition.

Quartz crystal is piezoelectric, that is, it can accumulate electrical charges across some planes when it is subjected to mechanical stress, like bending, and also it can be caused to bend if charges are placed across the crystal plane. Quartz is so stable that it can withstand all but the most extreme environmental condition without getting deformed. Its shape remains relatively the same with temperature fluctuations. This is why a quartz clock remains relatively accurate in every given temperature.

The quartz crystal in a quartz clock is designed in the form of a tuning fork. Electrical charges from the battery of a clock passing across the crystal plane sets it in motion and then the clock’s circuitry measures the current fluctuations that represent the resonate motions of the tines.

The quartz tuning forks are made so accurate that they vibrate at 32,768 times per second plus or minus one six-hundredth. In order to get the forks tuned to this frequency, the designers first add some deposits of gold at the ends of the tines to lower the fork’s vibration frequency. Then a laser zaps tiny bits of the gold off until the desired frequency is reached.

MECHANISM

I. The quartz oscillator receives an electrical charge from an integrated circuit, which gets its power from the watch battery (or, in the case of a battery-less watch, the poser storage cell). The electricity makes the quartz vibrate, or oscillate, at the rate or 32,768 times per second. (Quartz crystals can be cut to vibrate at a huge range of frequencies. the bigger the piece, the slower it vibrates.)

II. As the quartz oscillates, it sends electrical pulses- at the same rate of 32,768 per second- back to the integrated circuit. A device called a "trimmer" regulates the quartz oscillations.

III. The IC "divides the electrical pulses repeatedly until they have been reduced to a single pulse each second. The circuit is, in effect, counting the pulses and returning to zero each time the count hits 32,768.

IV. If the watch is an analog model, the one-second impulses are transmitted to a stepping motor, which transforms them into mechanical pulses that drive a chain of gears and, ultimately, the watch hands. If it's a digital watch, the integrated circuit continues counting pulses as they add up to minutes, hours, and if the watch has a calendar display, days. In a digital watch the IC also provides the power to the display and contains the watch's setting functions. Because a digital watch has no gears or other moving parts, its construction is called "solid state."

The basic formula for calculating the fundamental frequency (f) of vibration of a cantilever as a function of its dimensions (quadratic cross-section) is:
where

1.875 the smallest positive solution of cos(x)cosh(x) = -1
l is the length of the cantilever
a is its thickness along the direction of motion
E is its Young's modulus
and ρ is its density

A cantilever made of quartz (E = 10¹¹ N·m^−2^ = 100 GPa and ρ = 2634 kg·m^−3 with a length of 3 mm and a thickness of 0.3 mm has thus a fundamental frequency of around 33 kHz. The crystal is tuned to exactly 2¹⁵ = 32,768 Hz or runs at a slightly higher frequency with inhibition compensation (see below).

REFERENCES

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