Measurement is a most important part of the sciences. Not only do scientists use measuring instruments of many kinds, they also have been in the forefront of developing new measuring tools and standards.
SI Units
While some countries continue to use their own customary measuring units (in the United States we use pounds, feet, gallons, and so on), the International System of Units (SI Units, from the French Systéme International) has been adopted worldwide for both commercial and scientific purposes. The basic SI units include the following:
 Meter: length or distance
 Kilogram: mass, more commonly called "weight"
 Second: time
 Ampere: electric current
 Candela: light and other radiation
 Mole: comparing other substances to the molecular weight of a certain form of carbon
 Kelvin: temperature
From these basic units come many "derived units." An example is the measurement of area, for which the unit, called the square meter, is derived from the basic length unit or meter. Another example is the measurement of velocity, commonly called "speed" for which the unit, called meters per second, is measured from the basic meter traveled.
How to Measure
To measure something usually means taking a widely accepted tool, such as a meter stick, and comparing it with the thing to be measured. Practical measuring methods abound, ranging from astronomers' methods for estimating the Earth's distance in lightyears from celestial objects to micrometers and other tools used for measuring very small objects. Scientists are even finding ways to measure the sizes of atomic particles, their "spin," and other characteristics.
For this discussion of measurement, the metric system will be used in all examples. If you have a meter stick at hand for reference it will be useful. If you do not have one, the commonly available foot ruler with a metric scale along one side (about 30 centimeters, each divided into tenths or millimeters) will do.
Let's say that you want to measure the width of a room with a meterstick. You agree to measure it to the nearest millimeter (or 0.001 meter). If you are not familiar with measuring in the metric system, note that a millimeter is equal to roughly ^{1}/_{32}inch.
If the room you are measuring has a hard floor, you can make a pencil mark at the end of the meter stick each time you lay it out across the room. Or, if there is a carpet, you can stick a pin in it to mark the end of the stick. Let's say that you find there are five whole meters and a part of a meter that looks like figure 10.1 at the arrow point. This would read 5.823 meters.
Multiple Measurements
But one measurement is not enough if you intend to understand the problems of measurement. You should try to get an independent second measurement. To do this, have a friend make a measurement without telling him or her ahead of time what result you got.
Suppose your friend measures the room using the same tools and methods and gets a result of 5.834.You now have two measurements: 5.823 and 5.834 meters. Which is right? No one knows. What if you measure it again? Good, but would that prove to be the correct measurement any more than the first two? Not likely. No one can know the "true" measurement. We must face the problem that there is no perfect way to measure it. Two people will probably not measure anything, even the width of a room, exactly alike. Even the same person is not likely to get the same measurement twice, especially if one allows enough time between measurements to forget the first measurement.
Just as important, no two meter sticks or other measuring instruments are exactly alike, either. There is no such thing as a perfect measuring instrument, just as there is no perfect measurement.
To get as good a measurement as is reasonably possible of the width of the room, it is necessary to take several independent measurements. Let's say you end up with five different values: (1) 5.823, (2) 5.834, (3) 5.829, (4) 5.830, and (5) 5.825. Now we have a statistical question: Which one shall we choose to be the width of the room? If we take the arithmetical average, or mean, we get 5.828 meters. This average, we see, is not anyone of the values we got by actual measurement. Is it the "correct" measurement? All we can say is that it is probably very close to it.
If you were measuring the diameter of a marble with a micrometer, you would find the same problems. Several independent measurements would probably give you several different values. Again, no one would know which was the correct diameter. Once again, there would be a statistical problem in choosing a number to represent the diameter, the "true" diameter, which no one can know.
Scientists have understood this uncertainty about measuring for a long time. It seems to surprise others, however. Some people react to this discovery by saying, "Well, if I can't really know what the measurement is, why bother trying to make it exact?" The reason is that scientists and others who keep working out better ways to measure things are trying to communicate better. Reporting scientific findings to others is an important part of scientific method, as you know, and communication among scientists has been helped immeasurably by better and better systems of measurement.

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