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S inch as the length of three barley grains placed end-to- end. Henry VIII decreed several standards of weight, most notably the grain which is still in use. He decreed the pound to be the weight of 7000 grains of wheat, and today there are still 7000 grains in one pound. In 1965, the British government initiated the change to the metric system, and by 1975, the change was com- pleted. This leaves the United States the only major nation using the "English" system, now called the U.S. Customary System or simply the U.S. System. Use of the metric system in the United States was made legal by an act of Congress in 1866, but it has not been widely adopted by the public. Metric has long been used in the United States by the scientific community. Doctors write prescriptions in metric units, and the pharmacist fills them in metric units. The U.S. Military measures their weapons in millimeters. Photographic film comes in the familiar 8, 16, and 35 millimeter sizes. Many other exam- ples could be cited. In 1971, the National Bureau of Standards recom- mended to Congress that the United States should change to the metric system. This has not yet happened. The United States automotive industry is in a particular state of chaos in this regard, often having U.S. and metric bolts and fasteners on the same vehicle. Measurements on elevator layouts are given in the U.S. system, however many escalator prints are dimensioned in metric measurements. Metric size bolts are used in most escalator assembly and installation. Therefore it is important to learn both systems, and to learn how to con- vert from one system to the other. Eventually the com- plete change to metric will take place, and many of today's customary units will disappear the way of the ancient "cubit". U.S. Customary System The U.S. system of measurement is used on most lay- out drawings. Dimensions are shown in feet, inches, and fractions of an inch. On engineering drawings and some detail drawings, decimal parts of inches instead of frac- tional parts are used. Thus, 1/2 inch is shown .500 inch. It is well to memorize decimal equivalents of some of the major inch fractions shown in Table 1. American National Standards Institute (ANSI) publication B87.1 states that zero is not used ahead of the decimal in stat- ing fractional inch measurements. When referring to architect's drawings or general con- tractor's drawings, you will encounter another complica- tion. Civil engineers and surveyors divide the foot into ten parts instead of twelve. Although dimensions on building layouts are given in feet and inches, elevations are shown in feet and decimals. Building elevations are normally shown relative to Mean Sea Level (MSL). Thus in a typ- 7 1/32" = .03125" 1/16" = .0625 1/8" = .125" 3/16" = .1875" 1/4" = .250" 5/16" = .9375" 3/8" = .375" 7/16" = .4375" 1/2" = .500" 9/16" = .5625" 5/8" = .625" 11/16" = .6875" 3/4" = .750" 13/16" = .8125" 7/8" = .875" 15/16" = .9375" TABLE 1 Standard Fraction to Decimal Conversion ical building layout, first floor elevation might be shown as 420.5, second floor elevation might be 432.5, etc. Using the above example, you should be able to deter- mine the distance from the first floor to the second as 12 feet. Additional discussion of fractions and decimals in the U.S. system will be found in the sections on bolts, screws, and fasteners. Metric System In 1790, the French Academy of Sciences undertook a project to determine an invariable standard for all mea- sures and weights. By 1799, they had produced the metric system. Public resistance to change stalled the adoption of the metric system, so in 1840 the French Parliament passed a law ordering use of the metric system in all com- merce and trade. Originally the meter was defined as one ten-millionth of the distance from the North pole to the equator on the meridian running through Paris. Errors were soon detected in this reference, so the meter was redefined as the distance between two lines etched on a platinum- irridium bar constructed for this purpose. A platinum- irridium weight was also constructed, equal in mass to 1 kilogram. These standards are stored in a carefully con- trolled environment in Paris. In 1960, the nations of the world agreed on a major revision of the metric system. The new standard meter is defined in terms of the wavelength of the orange-red radi- ation of krypton-86 (a gas). Using this new standard, measurements accurate to one ten-millionth of an inch are possible. Such high precision is essential to the guidance systems of space vehicles and interplanetary research devices. Principal units of the metric system are the meter for length; gram for mass; and liter for volume. The metric system is easy to use because all of its units are multiples