STANDARDS OF LENGTH, WEIGHT, AND VOLUME IN THE UNITED STATES W&L~M ALBERTNOTES, U N I V ~ S I OF T YILLINOIS, URBANA, ILLINOIS
Even the majority of intelligent chemists do not seem to know that there is no standard yard or pound in use as a primary standard of length or weight in the United States. The ultimate standards for all measurements of length and weight are the international metric standards kept a t S h e s near Paris. There is no primary standard of volume in the world. Standard of Length
It was originally intended that the meter should be one ten-millionth of a quadrant of the earth's circumference measured on a meridian passing through the poles. The measurements undertaken to establish the length of the meter were not so accurate as other measurements since made and the actual meter now used everywhere in the world is referred to the distance between two fine lines near the ends of a bar of platinumiridium, measured with the bar in contact with melting ice. This bar is a t S h e s . The length of this bar in relation to the length of a similar bar kept a t the Bureau of Standards in Washington is known with an accuracy of about one part in 15,000,000. Therelation of this meter bar to the wavelength of the red line of the cadmium spectrum has now been very accurately determined and a t a meeting of the International Commission on Weights and Measures held in Paris in the fall of 1927 this wave-length was adopted as the standard of length. This gives a standard related to the meter bar a t S h e s as accurately as measurements can now be made but one which is independent of that meter and which can be reproduced whenever this is desired. The legal yard of the United States is derived from the meter a t S k e s by the statement that the meter is exactly 39.37 inches in length. The legal yard of Great Britian, established in 1855, is the distance between two lines on gold plugs set in a bronze bar kept a t London. The distance is measured a t 62OF. The lines are so poorly engraved, however, that it is impossible to measure the distance with a greater accuracy than one part in a million. As stated above, there is no standard yard at the Bureau of Standards or anywhere in the United States. Standard of Weight
It is customary to speak of standards of mass rather than standards of weight. Those who insist on this distinction (which is, of course, real and important, since weight varies with the place) seem to overlook the fact that accurate comparisons of masses are always made with a balance, which
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in reality compares weights and not masses. It is quite as logical to call the primary standard a standard of weight as a standard of mass. It was originally intended that the standard of weight in the metric system should be the weight, in a vacuum, of a cubic decimeter of water a t its maximum density, 4". As with the meter, later determinations have been made which were more accurate than those which established the weight of the first kilogram. According to these later determinations, the standard kilogram a t S k e s weighs 27 milligrams less than an actual cubic decimeter of water. The difference (about 1part in 40,000) seldom requires consideration, since most volumes are determined by the weight of the water required to fill them, as will be seen below. Forty kilogram weights were made and tested in France in accordance with a treaty signed by 17 nations on May 20, 1875. One of those was selected as the international standard and each of the others was tested with reference to this, and the small differences were determined with an accuracy of 0.01 to 0.02 milligram. No. 20 of the set was received by the President of the United States on Jan. 2,1890, and is kept a t the Bureau of Standards on a plate of crystal quartz, covered with two concentric bell jars. There is also a second "prototype" standard a t the Bureau. These standards were so much better than any other standards available at that time that in 1893 the office of Standard Weights and Measures decided that "greater stability in weights and measures as well as higher accuracy in their comparison" could he obtained by deriving from them not only other metric units, such as the gram and milligram, but also the various avoirdupois, troy, and apothecaries' units. The ratio of the avoirdupois pound to the kilogram was found to be: 1 avoirdupois pound = 0.4535924277 kilogram by the joint work, in 1883, of the British Standards Office and the International Bureau of Weights and Measures, and is the only legal standard for all weights used in this country. There are, however, a t the Bureau of Standards, four standards of 1 avoirdupois pound, two of 1 troy pound and one of 10 troy ounces, all derived from the standard kilogram. There are, also, still other standards derived from and based on these and used as "working standards." These "working standards" include test cars weighing, with weights, 100,000 pounds and used for testing railroad track scales.
Standard of Volume There is no concrete, accurate standard of volume in the United States, nor, indeed, anywhere in the world. The liter is arbitrarily defined as the volume occupied by one kilogram of water a t its maximum density, weighed in a vacuum. As stated above, one liter of water weighs 27
milligrams less than an actual cubic decimeter of water. The volume, which by common consent is called one cubic centimeter, holds 0.027 milligram less water than that contained in an actual cubic centimeter. For this reason, some are accustomed to speak of milliliters instead of cubic centimeters, but this usage has not been generally adopted. The difference between this volume and the volume of a real cuhic centimeter is only about 1 part in 40,000. It is very rarely important to remember that the volume which we call a "cubic centimeter" is arbitrarily referred to the volume occupied by one gram of water, as the liter is arbitrarily referred to the volume occupied by one kilogram of water. The U. S. gallon is, by definition, the volume of exactly 231 cubic inches. This volume of water weighs 58,285.9 grains a t 6Z°F., in air. It weighs 62.7 grains more in a vacuum. The British "Imperial gallon" is the volume occupied by 70,000 grains, or 10 avoirdupois pounds of water a t GZ°F., in air, under a pressure of 30 inches of mercury.
