| MadSci Network: General Biology |
This may be a long answer but the historical and current information presented may help answer your and other's questions. I have included the basic descriptions of various units used to help explain why one system may be more desirable than the other. The majority of the information presented was taken from: http://www.unc.edu/~rowlett/units/index.html All systems of weights and measures, metric and non-metric, are linked through a network of international agreements supporting the International System of Units. The International System is called the SI, using the first two initials of its French name Système International d'Unités. The key agreement is the Treaty of the Meter, signed in Paris on May 20, 1875. Forty-eight nations have now signed this treaty, including all the major industrialized countries. The United States is a charter member of this metric club, having signed the original document back in 1875. The SI is maintained by a small agency in Paris, the International Bureau of Weights and Measures (BIPM, for Bureau International des Poids et Mesures), and it is updated every few years by an international conference, the General Conference on Weights and Measures (CGPM, for Conférence Générale des Poids et Mesures), attended by representatives of all the industrial countries and international scientific and engineering organizations. As BIPM states on its web site, "the SI is not static but evolves to match the world's increasingly demanding requirements for measurement." At the heart of the SI is a short list of base units defined in an absolute way without referring to any other units. The base units are consistent with the part of the metric system called the MKS system. In all there are seven SI base units: 1. the meter for distance, 2. the kilogram for mass, 3. the second for time, 4. the ampere for electric current, 5. the kelvin for temperature, 6. the mole for amount of substance, and 7. the candela for intensity of light. Other SI units, called SI derived units, are defined algebraically in terms of these fundamental units. For example, the SI unit of force, the newton, is defined to be the force that accelerates a mass of one kilogram at the rate of one meter per second per second. This means the newton is equal to one kilogram meter per second squared, so the algebraic relationship is N = kgms2. Currently there are 22 SI derived units. They include: 1. the radian and steradian for plane and solid angles, respectively; 2. the newton for force and the pascal for pressure; 3. the joule for energy and the watt for power; 4. the degree Celsius for everyday measurement of temperature; 5. units for measurement of electricity: the coulomb (charge), volt (potential), farad (capacitance), ohm (resistance), and siemens (conductance); 6. units for measurement of magnetism: the weber (flux), tesla (flux density), and henry (inductance); 7. the lumen for flux of light and the lux for illuminance; 8. the hertz for frequency of regular events and the becquerel for rates of radioactivity and other random events; 9. the gray and sievert for radiation dose; and 10. the katal, a unit of catalytic activity used in biochemistry. Future meetings of the CGPM may make additions to this list; the katal was just added in 1999. The General Conference on Weights are Measures has replaced all but one of the definitions of its base (fundamental) units based on physical objects (such as standard meter sticks or standard kilogram bars) with physical descriptions of the units based on stable properties of the Universe. For example, the second, the base unit of time, is now defined as that period of time in which the waves of radiation emitted by cesium atoms, under specified conditions, display exactly 9 192 631 770 cycles. The meter, the base unit of distance, is defined by stating that the speed of light, a universal physical constant, is exactly 299 792 458 meters per second. These physical definitions allow scientists to reconstruct meter standards or standard clocks anywhere in the world, or even on other planets, without referring to a physical object kept in a vault somewhere. In fact, the kilogram is the only base unit still defined by a physical object. The International Bureau of Weights and Measures (BIPM) keeps the world's standard kilogram in Paris, and all other weight standards, such as those of Britain and the United States, are weighed against this standard kilogram. This one physical standard is still used because scientists can weigh objects very accurately. Weight standards in other countries can be adjusted to the Paris standard kilogram with an accuracy of one part per hundred million. So far, no one has figured out how to define the kilogram in any other way that can be reproduced with better accuracy than this. The 21st General Conference on Weights and Measures, meeting in October 1999, passed a resolution calling on national standards laboratories to press forward with research to "link the fundamental unit of mass to fundamental or atomic constants with a view to a future redefinition of the kilogram." The next General Conference, in 2003, will surely return to this issue. Following are the official definitions of the seven base units, as given by BIPM. 1. meter (m) distance "The metre is the length of the path traveled by light in vacuum during a time interval of 1/299 792 458 of a second." 2. kilogram (kg) mass "The kilogram is equal to the mass of the international prototype of the kilogram." 3. second (s) time "The second is the duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom." 4. ampere (A) electric current "The ampere is that constant current which, if maintained in two straight parallel conductors of infinite length, of negligible circular cross-section, and placed 1 metre apart in vacuum, would produce between these conductors a force equal to 2 × 10-7 newton per metre of length." 5. kelvin (K) temperature "The kelvin is the fraction 1/273.16 of the thermodynamic temperature of the triple point of water." 6. mole (mol) amount of substance "The mole is the amount of substance of a system which contains as many elementary entities as there are atoms in 0.012 kilogram of carbon 12. When the mole is used, the elementary entities must be specified and may be atoms, molecules, ions, electrons, other particles, or specified groups of such particles." 7. candela (cd) intensity of light "The candela is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540 × 1012 hertz and that has a radiant intensity in that direction of 1/683 watt per steradian." Now to explain the English Customary Weights and Measures. Here you will find that the English units are rich in history and customs. In all traditional measuring systems, short distance units are based on the dimensions of the human body. The inch represents the width of a thumb; in fact, in many languages, the word for "inch" is also the word for "thumb." The foot (12 inches) was originally the length of a human foot, although it has evolved to be longer than most people's feet. The yard (3 feet) seems to have gotten its start in England as the name of a 3-foot measuring stick, but it is also understood to be the distance from the tip of the nose to the end of the middle finger of the outstretched hand. Finally, if you stretch your arms out to the sides as far as possible, your total "arm span," from one fingertip to the other, is a fathom (6 feet). A lot of different units for a measure of length, wouldn’t you agree? Historically, there are many other "natural units" or English units of the same kind, including the digit (the width of a finger, 0.75 inch), the nail (length of the last two joints of the middle finger, 3 digits or 2.25 inches), the palm (width of the palm, 3 inches), the hand (4 inches), the shaftment (width of the hand and outstretched thumb, 2 palms or 6 inches), the span (width of the outstretched hand, from the tip of the thumb to the tip of the little finger, 3 palms or 9 inches), and the cubit (length of the forearm, 18 inches). And the different English customary units and their differences continue to grow. In Anglo-Saxon England (before the Norman conquest of 1066), short distances seem to have been measured in several ways. The inch (ynce) was defined to be the length of 3 barleycorns, which is very close to its modern length. Remember that inch was also defined as the width of a thumb. The shaftment was also frequently used, but it was roughly 6.5 inches long. Several foot units were in use, including a foot equal to 12 inches, a foot equal to 2 shaftments (13 inches), and the "natural foot" (pes naturalis, an actual foot length, about 9.8 inches). The fathom was also used, but it did not have a definite relationship to the other units. You can see how quickly defining a foot could be come in this time period. When the Normans arrived, they brought back to England the Roman tradition of a 12-inch foot. Although no single document on the subject can be found, it appears that during the reign of Henry I (1100-1135) the 12-inch foot became official, and the royal government took steps to make this foot length known. A 12-inch foot was inscribed on the base of a column of St. Paul's Church in London, and measurements in this unit were said to be "by the foot of St. Paul's". Henry I also appears to have ordered construction of 3-foot standards, which were called "yards," thus establishing that unit for the first time in England. William of Malmsebury wrote that the yard was "the measure of his [the king's] own arm," thus launching the story that the yard was defined to be the distance from the nose to the fingertip of Henry I. In fact, both the foot and the yard were established on the basis of the Saxon ynce, the foot being 36 barleycorns and the yard 108. Meanwhile, all land in England was traditionally measured by the gyrd or rod, an old Saxon unit probably equal to 20 "natural feet." The Norman kings had no interest in changing the length of the rod, since the accuracy of deeds and other land records depended on that unit. Accordingly, the length of the rod was fixed at 5.5 yards (16.5 feet). This was not very convenient, but 5.5 yards happened to be the length of the rod as measured by the 12-inch foot, so nothing could be done about it. In the Saxon land-measuring system, 40 rods make a furlong (fuhrlang), the length of the traditional furrow (fuhr) as plowed by ox teams on Saxon farms. These ancient Saxon units, the rod and the furlong, have come down to us today with essentially no change. Longer distances in England are traditionally measured in miles. The mile is a Roman unit, originally defined to be the length of 1000 paces of a Roman legion. A "pace" here means two steps, right and left, or about 5 feet, so the mile is a unit of roughly 5000 feet. For a long time no one felt any need to be precise about this, because distances longer than a furlong did not need to be measured exactly. It just didn't make much difference whether the next town was 21 or 22 miles away. In medieval England, various mile units seem to have been used. Eventually, what made the most sense to people was that a mile should equal 8 furlongs, since the furlong was an English unit roughly equivalent to the Roman stadium and the Romans had set their mile equal to 8 stadia. This correspondence is not exact: the furlong is 660 English feet and the stadium is only 625 slightly-shorter Roman feet. In 1592, Parliament settled this question by setting the length of the mile at 8 furlongs, which works out to 1760 yards or 5280 feet. This decision completed the English distance system. Since this was just before the settling of the American colonies, British and American distance units have always been the same. However, other measurements were not standardized and differences can bee seen in these measurements; e.g. weight and volume Now for the bottom line to English customary units. Because of their many eccentricities, English customary units clearly are more cumbersome to use than metric units in trade and in science. As metrication proceeds, the English units are less and less in use. On the other hand, these traditional units are rich in cultural significance. We can trace their long histories in their names and relationships. We should not forget them, and it is unlikely that we will, even when Britain and America complete their slow conversion to the metric system. The American economy of the 22nd Century may be completely metric, but probably Americans will still call 30 centimeters a "foot" and 1600 meters a "mile." And how does the use of the metric system over customary English units compare to trades individuals? Many trades people use only linear measures; therefore, the change is an easy and positive one for them - from three kinds of units (feet, inches, and inch-fractions) to one (millimeters). A metric tape measure usually is the only new tool they require and extensive classroom work is rarely needed to convert inches to feet to yards, etc. Plumbing and HVAC personnel must learn the additional metric measures for mass, volume, pressure, force and temperature; however, most seem to welcome the change to a simpler, decimal-based system. Electricians, of course, have always worked in the metric world of volts, amps, and watts. Thus far, I’ve only identified two websites which I thought offered great additional information on measurements: http://www.unc.edu/~rowlett/units/index.html http://ts.nist.gov/ts/htdocs/200/202/mpo_edulinks.htm The following website offers a project to help learn the benefits and drawbacks to each measurement system: http://www.scs.k12.tn.us/STT99_WQ/STT99/Germantown_HS/folisd/metrichome.htm For even more information, search the Internet using the following key words: “Metric System History.” I found many reasonable articles and references discussing and comparing the various measurement systems.
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