Weights and measures are tools that we use to measure the physical properties of things. They are essential elements for trade and commerce, and they lie at the foundation of science and engineering.
A measurement unit is a precisely defined quantity that can be used to measure all other quantities of the same kind. For example, we can use the unit meter to measure any length or distance—the height of a person, the length of a swimming pool, or the distance to the moon.
A measurement standard is an object that uniquely defines or represents the size of a unit. One important standard was known as the international prototype kilogram. This standard was a particular weight that is made of an alloy (mixture) of the metals platinum and iridium. It was the standard for the kilogram, a unit of mass (amount of matter). All standard weights in the world, including those for pounds and ounces, were related to this one.
Beginning in the mid-1900’s, measurement standards for many units were redefined using values derived from universal natural phenomena. In 1960, the platinum-iridium bar that served as the measurement standard for the meter was replaced. Today, scientists define the meter as the distance traveled by light in a vacuum in 1/299,792,458 of a second. In 2018, the measurement standard for the kilogram was redefined as a value based on a universal fundamental measure known as Planck’s constant. This value cannot change over time.
All countries enforce the uniform use of measurement standards within their borders. Inspectors check the accuracy of gasoline pumps to make sure that what is being sold as a liter or a gallon of gasoline is actually that much gasoline. Other officials check the scales used to weigh meat, fruits, and vegetables in grocery stores. Still others weigh samples of packaged foods and compare the weight with what is printed on the labels.
Measurement systems.
Groups of units are organized into measurement systems. The most widely used measurement system is the modern metric system, which is known as the International System of Units. People commonly refer to this system by the initials SI, which stand for its name in French: Systeme International d’Unites.
Since about 1980, every technologically advanced nation but the United States has used SI for almost all measurements. In the United States, scientists and most engineers use SI. But for many commercial and everyday measurements, Americans use the inch-pound system. This system consists mainly of units that English people brought to what is now the United States. The English brought the units with them in the 1600’s, when they began to settle along the Atlantic coast of North America.
Because of the source of the units, Americans often call them English units. Because the units have been used for so long in the United States, people there sometimes refer to them as customary units.
The inch-pound units are now defined in terms of SI units. In 1959, the United States and other English-speaking nations entered into an agreement that made the foot equal to exactly 0.3048 meter and the pound equal to exactly 453.59237 grams.
Representatives of countries throughout the world regulate SI. These delegates gather every four years at a General Conference on Weights and Measures. A permanent organization called the International Bureau of Weights and Measures (BIPM) operates under the authority of the General Conference. The BIPM, which is located in France, maintains the fundamental measurement standards of the SI. For example, the BIPM carefully preserved the international prototype kilogram. This organization also decides the value of the physical constants used for measurement standards.
In the United States, the National Institute of Standards and Technology, an agency of the Department of Commerce, maintains the country’s primary measurement standards. That agency also compares other standards with the primary standards, and it conducts studies in precision measurement. Many different federal and state agencies carry out inspections to ensure fair measurements in commerce.
Conversion factors.
Tables with this article define commonly used units in the SI and inch-pound systems. The tables also provide factors that are needed to convert from one system to the other. Suppose, for example, you wanted to convert a distance of 15 miles to kilometers. You would multiply 15 by 1.609, the number of kilometers that make up 1 mile: 15 X 1.609 = 24.135. You would then round this number to 24. So 15 miles equals 24 kilometers.
Almost all the conversion factors in the tables are given in four significant figures. The significant figures are the digits that follow any zeroes at the beginning of a factor. For example, in the conversion factor 0.03937, used to convert millimeters to inches, the digits, 3, 9, 3, and 7 are significant figures.
The number of significant figures in a conversion factor indicates the precision of the factor—the larger the number of significant figures, the more precise the factor. A precision of four significant figures is sufficient for almost all practical purposes. The tables give factors for the teaspoon, the tablespoon, and the cup in fewer significant figures because people do not use those units for precise work. The factor for converting inches to centimeters is given in three figures because this conversion is exact.
Comparison of measurement systems.
The metric system is easier to use than the inch-pound system. Indeed, the metric system was designed for ease of use, with simple relationships established among the units. By contrast, the inch-pound system is a combination of groups of units that developed separately. As a result, the inch-pound system involves a host of conversion factors that are difficult to use.
One feature that makes SI easy to use is the decimal relationship between the units for measuring a given property. For example, the base unit for the measurement of length is the meter. A larger unit for measuring this property is the dekameter, which is 10 times as long as the meter. The hectometer is 10 times as long as the dekameter, and the kilometer is 10 times as long as the hectometer.
To convert from one of these units to another, merely shift the decimal point the appropriate number of places to the left or right. For example, to convert meters to kilometers, shift the decimal point three places to the left. The three decimal places represent the three factors of 10 by which the meter differs from the kilometer. Thus, 2,864 meters converts to 2.864 kilometers.
Another feature that makes SI easy to use is the simple relationship between groups of units that measure different properties. For example, 1 liter of water has a volume of 1,000 cubic centimeters and a mass of 1 kilogram.
In the inch-pound system, the unit that corresponds to the liter is the quart. But people in the United States use two kinds of quarts—one to measure “dry” products, such as strawberries, and the other for liquids. The quart used for dry measure has a volume of 67.20 cubic inches. The quart used for liquid measure has a volume of 57.75 cubic inches—and 1 quart of water weighs 2.086 pounds.
Development of units of measure.
No one knows who developed the first measurement units. But surely the need existed when tribes began to trade. Suppose two tribes regularly exchanged ropes for grain. The owners of the grain would want to know how long the ropes would be. The tribes would therefore need to agree on a unit of length. The owners of the ropes would want to know how heavy the grain was or how many baskets or pots it would fill. Thus, the two parties would also need a unit of weight or volume.
Early units.
The earliest measurement units were probably units of length. These units almost certainly were related to parts of the body—the hand or foot, for example. Later, local leaders established a particular stick of wood or a certain rod of metal as a standard. They made copies of this standard for commercial use and placed the primary standard in a temple or some other secure place. People based early units of weight on the heaviness of a given number of kernels of grain. They based units of volume on the capacity of baskets and, later, clay pots.
Ancient civilizations developed simple groups of standard units. In an old Hebrew system, for example, four finger widths made a palm, and six palms made a cubit. Not all cubits were the same, however. The Hebrew cubit was about 450 millimeters long. The cubit that the Egyptians used in building the Pyramids of Giza in about 2600 to 2500 B.C. was about 524 millimeters in length.
Inch-pound units.
Some inch-pound units originated in the Roman Empire, which once included most of what is now England. The mile, for example, comes from mille passus, which means thousand paces in Latin, the language of the empire. Other inch-pound units reached England when Norman invaders from what is now France conquered England in 1066.
In the 1500’s, a law signed by Queen Elizabeth I of England established many of the present relationships among the inch-pound units. Previously, the mile of 1,000 paces had been understood to be 5,000 feet. The new law established the statute mile of 5,280 feet. This change also made the mile equal to exactly 8 furlongs. The furlong was originally the length of a furrow that oxen could plow without resting. The change was convenient because the furlong was still widely used at the time. It is now used mainly to measure distances in horse races.
The British government simplified the standards for volume measurement in 1824. It established an Imperial gallon, defined as the volume occupied by 10 pounds of water. The new measures of the Imperial system were adopted throughout the British Empire, but not in the United States, which retained the old liquid and dry pints and quarts that had arrived with the colonists.
The metric system.
In the 1600’s and 1700’s, a rise in international commerce and an increase in international contact among scientists created a need for a new system. Merchants and scientists wanted a system that would be widely agreed upon and easy to use. Various proposals were debated. Finally, in 1790, France’s National Assembly, or parliament, asked the French Academy of Sciences to propose a system of measurement that would be suitable for adoption by the entire world. The work of the Academy resulted in the metric system adopted by France in 1795.
France required the French people to use the metric system in 1840. Soon after, other countries in Europe, Central America, and South America began to use the system. In 1875, 17 nations, included the United States, signed a document known as the Treaty of the Meter. That document established the International Bureau of Weights and Measures. It also ensured continuing international cooperation in the development of metric weights and measures.
By 1900, scientists throughout the world were using metric units, and most of the technologically advanced countries had adopted the metric system for everyday use. The main exceptions were the United States and the nations that were then part of the British Empire. But in 1965, the pressures of international commerce persuaded the United Kingdom to announce that it would convert to SI. New Zealand began conversion in 1969, Australia in 1970, and Canada in 1975.
In the United States, Presidents John Adams and Thomas Jefferson, whose terms ran from 1797 to 1809, had shown interest in the metric system. However, it was still little used at that time, even in France. The United States Congress took no action on the metric system until 1866, when it legalized use of the metric units. In 1893, the U.S. Office of Weights and Measures carefully compared the U.S. prototype standards with the metric standards in France. The Americans then defined the U.S. yard in terms of the meter and the U.S. pound in terms of the kilogram.
In 1975, the United States Congress passed the Metric Conversion Act, which called for a voluntary changeover to the metric system. A bill passed in 1988 declared that the metric system was “the preferred system of weights and measures for United States trade and commerce.” However, the bill did not establish a timetable for conversion from inch-pound units, nor did it specify penalties for failing to convert.
In the early 2000’s, international trade and other international business were becoming more important to the United States. The growth of this commerce increased the pressure to use metric units in the United States. In engineering and manufacturing, conversion to the metric system was proceeding steadily. More and more consumer products were being sold in metric packaging. Examples included all medicines, even aspirin and vitamins; wine and liquor; and many groceries and soft drinks.
