International System of Weights and Measures denned the second to be " 1 / 31,556,925.9747 for the tropical year 1900 January 0 at 12 hours ephemeris time."*4 In 1960 this standard was accepted by the General Conference with the caveat that work continue toward development of an atomic clock for the accurate measurement of time.

The atom exhibits very regular, hyperfine energy-level transitions and it is possible to count these "cycles" of energy. In 1967 the General Conference accepted 9,192,631,770 cycles of cesium-133 as the measurement of one second, making the atomic clock the true international timekeeper. The cesium clock is maintained in Boulder, Colorado, in the offices of the National Institute of Standards and Technology (formally the National Bureau of Standards). Its accuracy is one part in 1,000,000,000,000 (1012). It will not gain or lose a second in 6000 years.

2.1.4 The Use of Prefixes in the Metric System

One of the strengths of the metric system is the consistency of its terms. In other measurement systems, the names for measures change as the size of the measurements change. For instance, consider the changes in the English measurement system for length (inch-foot-yard), weight (ounce-pound-ton), and volume (ounce- quart-gallon). In the metric system, all measurement names consist of a root term that, by use of prefixes, yields fractions and multiples of the base measurement unit.

The metric system is a decimal system, based on powers of 10. Table 2.5 is a list of the prefixes for the various powers of 10. Between scientific notation and the prefixes shown below, it is very simple to identify, name, read, and understand 36 decades of power of any given base or derived unit.

2.1.5 Measurement Rules

There are several general rules about making measurements. These rules are standard regardless of the type of equipment being used, material being studied, or the measurement units being used.

1. The quality of the measurement is only as good as the last clearly read number. No matter how obvious an estimated number may seem, it is not as reliable as the one your equipment actually provides for you. For example, see Fig. 2.1 and Fig. 2.2. If you want greater accuracy in your readings, you need better equipment, which provides more precise measurements. One safe way to indicate greater accuracy is to qualify a number. In the case of Fig. 2.1, you could write 6.2 + 0.2. The selection of ±0.2 is somewhat arbitrary, as the length is obviously greater than 6.0, yet smaller than 6.5.

Ephemeris time is a uniform measure of time defined by the orbital motions of the planets.

Using this ruler, it is correct and accurate to measure the block at 6.2 units.

Fig. 2.2 It is important to read only to the limits of your equipment.

4.Be sure the equipment is used in the proper environment. Some equipment is designed to be used in limited environmental conditions. For example, a vacuum thermocouple gauge is calibrated for readings made in specific gaseous environments. If the vacuum system contains gases other than those the gauge was designed to read, inaccurate pressure data will result. In addition, variables such as temperature, elevation, drafts, and whether the equipment is level can all affect accurate performance.

5.Be aware of user error. Many errors are caused by the operator, through sloppy readings or lack of experience with particular equipment. Be sure you know how to read your equipment and how to make any necessary conversions of your readings to the units that you are using (i.e., microns to torr).

6.Avoid parallax problems. A "parallax" problem occurs when the object being observed is not placed directly between the eye and measuring device, but rather at an angle (see Fig. 2.3) between the two. An incorrect sighting is easy to avoid, but requires conscious effort. Parallax problems show up with linear as well as liquid measurements.

7.Make many readings. This form of cross-checking proves that your work is reliable and reproducible. It is unscientific to make conclusions based on a limited amount of data. On the other hand, there has

Fig. 2.3 Correct and incorrect sightings of an object.