In every scientific discipline, scientists must have a way to make quantitative observations that can be communicated to other scientists. These quantitative observations are normally called measurements. Familiar measurements include distance, volume, mass, time, and temperature.
In order for the measurements to be understood by anyone and everyone, there must be a system that defines the size of standard units of measurement. The size of a given standard unit is arbitrary, but it works as long as everyone agrees on what it is. The standard unit of length, for example, is called a meter (m), and it is defined as the length of the path traveled by light in a vacuum during a time interval of 1/299,792,458 of a second. There is no particular reason requiring a meter to be this length; it could just as easily be defined as the distance light travels in 1/300,000,000 of a second. The key is that the scientific community must agree on what length will be called 1 meter and that there be an objective, unchanging reference for that distance. In this way, scientific measurements can be shared and communicated, understood by all.
For example, the length of a meter was previously defined as the distance between two lines marked on a bar of platinum-iridium alloy kept at the International Bureau of Weights and Measures (BIPM, Bureau International des Poids et Mesures) in France. The rod was kept in a carefully controlled atmosphere, supported on cylinders placed exactly 571 mm from each other. These precautions were taken to preserve the bar and keep it from being deformed in any way, because any change to the length of the rod would have then redefined the length of a meter. Changing the reference standard to be based on the speed of light in a vacuum is a more secure way to ensure that the definition of a meter does not change.
Systems of Measurement
A set of units in which some units are defined by their relationship to other units in the system and not by a physical standard is called a system of measurement. There are many systems of measurement in use today. One familiar system is the British Imperial System, which contains units such as the inch (distance), the pound (weight), and the gallon (volume). The British system evolved from units people used as far back as the Middle Ages, which were based on references handy to them. The foot, for example, is so named because it was supposed to be equal to the length of a human foot.
By the 19th century, it was realized that the system needed to be standardized, and Britain passed the Weights and Measures Act of 1824, which created precise and objective definitions. The definitions were updated in 1963, so today an imperial gallon, for example, equals the space occupied by 10 pounds of distilled water at a density of 0.998859 g/mL.
Although its units are now precisely defined, the British Imperial System is not a good choice for scientific measurements. The units are related to one another in inconsistent ways. For example, 1 mile = 1760 yards, 1 yard = 3 feet, and 1 foot = 12 inches. These relationships are difficult to remember and tricky to use in calculations. A better system would have consistent relationships and names that follow simple rules. Fortunately, this system exists. The International System of Units (SI), which in French is Système Internationale d'Unités, is the system of units used by the global scientific community, based on seven fundamental units. A more general form of the International System of Units is the metric system. It improves on the Imperial British System because the units within a particular type of measurement, such as length or mass, are all based on powers of 10 and are therefore more easily related to each other.
SI Base Units
|Mole||mol||Quantity of matter|
Examples of Derived SI Units with Names
|Volt||V||Electric potential difference|
|Larger than the Base Unit|
|Smaller than the Base Unit|
Mass, Weight, and Volume
The International Prototype of the Kilogram
Two familiar temperature scales in everyday use are the Fahrenheit and Celsius scales. The Fahrenheit temperature scale is based on a freezing point of water of 32°F and a boiling point of water of 212°F at sea level. The interval between the freezing point of water and the boiling point of water is divided into 180 parts, and the size of each part is equal to one Fahrenheit degree. In developing the scale, the Polish-Dutch physicist Daniel Gabriel Fahrenheit (1686–1736) originally designated the freezing point of water to be 30°F and normal human body temperature to be 90°F, but based on more accurate measurements, these values eventually were adjusted.The Celsius temperature scale is based on a freezing point of water of 0°C and a boiling point of water of 100°C at sea level. The interval is divided into 100 parts so that each part is equal to one Celsius degree. Because there are fewer divisions between the freezing and boiling points of water on the Celsius scale than on the Fahrenheit scale, a Celsius degree is larger than a Fahrenheit degree. Although the Fahrenheit scale is commonly used in the United States, the Celsius scale is appropriate for scientific work. Conversions between the two scales are made according to the following equations. (Note: 5/9 is a reduced fraction of 100/180, which accounts for the different size of a Fahrenheit degree and a Celsius degree.)