To determine the error associated with a measurement, scientists often refer to the precision and accuracy of the measurement.

A

The precision of an experiment is a measure of the reliability of the experiment, or how reproducible the experiment is. In this figure, we see that the marksman's instrument was quite precise, since his results were uniform due to the use of a sighting scope. However, the instrument did not provide accurate results since the shots were not centered on the target's bull's eye. The fact that his results were precise, but not accurate, could be due to a misaligned sighting scope, or a consistent operator error. Therefore precision tells us something about the quality of the instrument's operation.

B

The accuracy of an experiment is a measure of how closely the experimental results agree with a true or accepted value. In this figure, we see a different experimental result. Here, the shots are centered on the bull's eye but the results were not uniform, indicating that the marksman's instrument displayed good accuracy but poor precision. This could be the result of a poorly manufactured gun barrel. In this case, the marksman will never achieve both accuracy and precision, even if he very carefully uses the instrument. If he is not satisfied with the results he must change his equipment. Therefore accuracy tells us something about the quality or correctness of the result.

C

As scientists, we desire our results to be both precise and accurate. As shown in this figure, the shots are all uniform and centered on the bull's eye. This differs from the first figure in that the marksman has compensated for the poorly aligned sighting scope.

D

One benefit of taking many measurements of a single property is that blunders are easily detected. In the figure below we see that the results are both accurate and precise with the exception of an obvious blunder. Because several measurements were made, we can discount the errant data point as an obvious mistake, probably due to operator error.

Picture 1 picture 2 picture 3 picture 4

Retell text B.

Unit 6

TIME MEASUREMENT

1. Practice reading the following words.

Unique, periodic, ambiguous,feature, pendulum, oscillator, circuit, precisely, cesium, hydrogen, pendulum, torsion, ephemerid, terrestial, nitrogen, celestial.

1. Discuss with a partner the role of clocks in everyday life.
2. Read and translate text A.
3. Find in the text English equivalents to the following words and word combinations.

Істинний час, інтервал часу, періодична подія, шкала (масштаб) часу, одиниця часу, кварцовий осцилятор, наручний годинник, фактор навколишнього середовища, застосування, дискретне значення, час розігріву.

TEXT A

Time measurements can be divided into two general categories. The first category is time-of-day measurements. Time-of-day is labeled with a unique expression containing the year, month, day, hour, minute, second, etc., down to the smallest unit of measurement that we choose. When we ask the everyday question, “What time is it?”, we are asking for a time-of-day measurement. The second type of time measurement (and the one more commonly referred to by metrologists) is a time interval measurement. A time interval measurement requires measuring the interval that elapses between two events. Time interval is one of the four basic standards of measurement (the others are length, mass, and temperature). Of these four basic standards, time interval can be measured with the most resolution and the least amount of uncertainty. Time keeping involves both types of measurements. First, we must find a periodic event that repeats at a constant rate. For example, the pendulum in a clock may swing back and forth at a rate of once per second. Once we know that the pendulum swings back and forth every second, we can establish the second as our basic unit of time interval. We can then develop a system of timekeeping, or a time scale.

A time scale is an unambiguous way to order events. It is created by measuring a small time unit (like the second) and then counting the number of elapsed seconds to establish longer time intervals, like minutes, hours, and days. The device that does the counting is called a clock. There are many types of periodic events that can form the basis for many types of clocks.

All clocks share several common features. Each clock has a device that produces the periodic event mentioned previously. This device is called the resonator. In the case of the pendulum clock, the pendulum is the resonator. Of course, the resonator needs an energy source, a mainspring or motor, for example, before it can move back and forth. Taken together, the energy source and the resonator form an oscillator. Another part of the clock counts the “swings” of the oscillator and converts them to time units like hours, minutes, and seconds, or smaller units like milliseconds (ms), microseconds (ms), and nanoseconds (ns).

And finally, part of the clock must display or record the results. The frequency uncertainty of a clock’s resonator relates directly to the timing uncertainty of the clock.

Quartz oscillators are used extensively in wristwatches, wall and desk clocks, and electronic circuits. The resonance frequency of quartz relies upon a mechanical vibration that is a function of the size and shape of the quartz crystal. No two crystals can be precisely alike or produce exactly the same frequency. Quartz oscillators are also sensitive to environmental parameters like temperature, humidity, pressure, and vibration. These shortcomings make quartz clocks inadequate for many applications, and led to the development of atomic oscillators.

Atomic oscillators use the quantized energy levels in atoms and molecules as the source of their resonance frequency. The laws of quantum mechanics dictate that the energies of a bound system, such as an atom, have certain discrete values. An electromagnetic field can boost an atom from one energy level to a higher one. Or an atom at a high energy level can drop to a lower level by emitting electromagnetic energy. The resonance frequency (f ) of an atomic oscillator is the difference between the two energy levels divided by Planck’s constant (h) Time is kept by observing and counting the frequencies at which electromagnetic energy is emitted or absorbed by the atoms. In essence, the atom serves as a pendulum whose oscillations are counted to mark the passage of time.

The least expensive and most common type is the rubidium oscillator, based on the 6.835 GHz resonance of Rb87. Rubidium oscillators range in price from about $2000 to $8000. They are well-suited for applications that require a small, high-performance oscillator with an extremely fast warm-up time.

The second type of atomic oscillator, the cesium beam, serves as the primary reference for most precision timing services. The resonance frequency of cesium (9.1926 GHz) is used to define the SI second. The price of a cesium oscillator is high, ranging from about $30,000 to $80,000.

A third type of atomic oscillator, the hydrogen maser, is based on the 1.42 GHz resonance frequency of the hydrogen atom. Although the performance of hydrogen masers is superior to cesium in some ways, they are not widely used due to their high cost. Few are built, and most are owned by national standards laboratories. The price of a hydrogen maser often exceeds $200,000.

1. Answer the following questions.
1. What is the time interval measurement?
2. What is the difference between the time interval and the other three basic standards of measurement?
3. What types does a time keeping involve?
4. What time units do you know?
5. Can the resonator work without any energy source?
6. Where are quartz oscilators used?
7. Compare three basic types of atomic oscillators.