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.