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Number of Clock Movements vs Accuracy and Reliability
A clock is an instrument for measuring time. The part of the clock that ticks is called escapement. The efficiency of an escapement's design, that is, how much energy is converted into resonant motion, directly affects the accuracy of a clock, and how long a clock can operate between windings. Clock movement, therefore, largely depends on the condition of the escapement.
The verge escapement is the earliest known type of escapement, the mechanism in a clock that regulates the swinging of a pendulum for accurate timekeeping. A clock is also made more efficient by using rubies as jewel bearings for the axes
and the parts of the escapement that make contact repeatedly. The rubies are hard and do not wear to the same extent as metal. They also introduce accurately predictable amounts of friction that can be compensated for in the design. More complicated movements typically have more jewel bearings.
An especially accurate clock is called a chronometer. A chronometer's escapement is usually designed to minimize the energy and time required to unlock the escapement, so that it affects the resonant frequency of the oscillator as little as possible.
An ideal clock is a scientific principle that measures the ratio of the duration of natural processes. The recurrent, periodic process (a metronome) is an oscillator and typically generates a clock signal. But the signal alone is not "the clock." It is the whole system and process that includes the counter, its indicator, and everything else supporting it.
An ideal clock involves a recurrent periodic process and a counter. A good clock is one which, when used to measure other recurrent processes, finds many of them to be periodic.
Early clocks were powered by heavy weights attached to long chains. Every day the weight was returned to the top of the clock, and throughout the day gravity pulled the weight down, thereby causing the gears to move. Unfortunately, this
only worked if the clock was mounted vertically and there was room for the weights to hang down.
The invention of the mainspring enabled clocks to be portable and eventually gave rise to what we call a pocket watch today. One problem with early mainsprings, though, was that as the spring wound down it lost power, and as a result the watch or clock would get slower and slower as the day progressed.
Modern clocks define constant units of time: an hour is always sixty minutes, of sixty seconds each. The number of reliable digits in time is called significant digits. Precision is measured in terms of these significant digits. A wristwatch does just that - 6 digits of precision (24 hours, 60 minutes, 60 seconds). The units of time determine the number of clock movements.
The terms accuracy and reliability are used interchangeably by most non-scientists and are even listed by many dictionaries as largely synonymous. In their scientific usage, however, these terms have specific and important distinctions.
The clock's accuracy and reliability comes in when the successive levels of smaller and smaller error tolerances are considered. Within current measurement tolerances, they all beat in a manner such that if one is chosen as periodic then the others are all deemed to be periodic also.
Clock accuracy is defined as the level of agreement of the frequency of a clock with the ideal frequency. This is specified as the magnitude of the fractional frequency offset from the ideal frequency. On the other hand, clock reliability is
achieved when clock skew, clock delay, clock area and power, and noise are minimized.
Accuracy and reliability can be best summed up in precision and is best explained through a time analogy. When someone asks, "What time is it?," the answer can vary in precision from 8:02:57 to 8:03 (rounded to the minute) to "about 8:00" (rounded to the hour). Likewise, if someone told you the time was "about 8:00," you could not derive any more precision, such as 8:02, 8:04 or 8:01.
The context in which you use precision also makes a difference. High precision is mandatory in timing certain sporting events as time needs to be measured in 10ths or 100ths of seconds. Rounding to the nearest hour would result in a lot of ties.
Accuracy is the extent to which the measurements are a reliable estimate of the ‘true' value. Both random errors and systematic biases reduce accuracy.
Reliability is a more subjective term, referring usually to interpretations but sometimes to measurements. Reliability is affected by both precision and accuracy. Dubious assumptions, regardless of measurement precision and accuracy make interpretations unreliable.
Accuracy refers to the agreement between a measurement and the true or correct value. If a clock strikes twelve when the sun is exactly overhead, the clock is said to be accurate. The measurement of the clock (twelve) and the phenomena it is meant to measure (The sun located at zenith) are in agreement. Accuracy cannot be discussed meaningfully unless the true value is known or is knowable.
The idea to get used to here is that accuracy only refers to the agreement between the measured value and the expected value and that this may or may not say something about the quality of the measuring instrument. A stopped clock is accurate at least once each day.
In summary, precision is higher than accuracy because accuracy is affected by both precision and systematic biases. Accuracy is higher than reliability because reliability is affected by measurement accuracy.
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