tional information on this subject.

3.2. Standards of Mass

The primary standard of mass for this country is United States Prototype Kilogram 20, which is a platinum-iridium standard kept at the National Bureau of Standards. The value of this mass standard is known in terms of the International Prototype Kilogram, a platinumiridium standard which is kept at the International Bureau of Weights and Measures.

For many years the British standards were considered to be the primary standards of the United States. Later, for over 50 years, the U. S. avoirdupois pound was defined in terms of the Troy Pound of the Mint, which is a brass standard kept at the United States Mint in Philadelphia. In 1911 the Troy Pound of the Mint was superseded, for coinage purposes, by the Troy Pound of the National Bureau of Standards. Since 1893 the avoirdupois pound has been defined in terms of the United States Prototype Kilogram 20 by the relation:

1 avoirdupois pound=0.453 592 427 7 kilogram.

Insofar as can be determined, these changes in definition have not made any change in the actual value of the pound.

The grain is 1/7 000 of the avoirdupois pound and is identical in the avoirdupois, troy, and apothecaries systems. The troy ounce and the apothecaries ounce differ from the avoirdupois ounce but are equal to each other, and equal to 480 grains. The avoirdupois ounce is equal to 4371⁄2 grains.

In Great Britain the Imperial Pound, an avoirdupois pound, is represented by a physical standard made in 1845. According to the most recent published determination

1 British Imperial Pound=0.453 592 34 kilogram.

There is substantial evidence that the British standard has diminished by about 1 part in 5 million since 1883 in relation to the kilogram, and is therefore smaller than the U. S. pound by that amount.

a. Distinction Between Mass and Weight

The mass of a body is a measure of the quantity of material in the body. The weight of a body is defined as the force with which that body is attracted toward the earth. Confusion sometimes arises from the practice of referring to standards of mass as "weights" and from the fact that such standards are compared by "weighing" one against another by means of a balance. Standard "weights" are, in reality, standards of mass.

Another practice which tends to confusion is that of using the terms kilogram, gram, pound, etc., in two distinct senses; first, to designate units of mass, and second, to designate units of weight or force. For example, a body having a mass of one kilogram is called a kilogram (mass) and the force with which such a body is attracted toward the earth is also called a kilogram (force).

The International Kilogram and the U. S. Prototype Kilogram are specifically defined by the International Conference on Weights and Measures as standards of mass. The U. S. pound, which is derived from the International Kilogram, is, therefore, a standard of mass.

So long as no material is added to or taken from a body its mass remains constant. Its weight, however, varies with the acceleration of gravity, g. For example, a body would be found to weigh more at the poles of the earth than at the equator, and less at high elevations than at sea level. (Standard acceleration of gravity, adopted by the International Committee on Weights and Measures in 1901 is 980.665 cm/sec2. This value corresponds nearly to the value at latitude 45° and sea level.)

The most accurate of the precision balances at NBS is used to compare secondary and laboratory standards of mass with the national prototype kilogram. This balance is operated by remote control in order to avoid errors caused by the heat given off by the body of the observer. By means of rods (above and lower right) the balance pan is arrested and released, the loads on the pans are interchanged, and sensitivity weights are added. A precision better than 1 part in 100 000 000 is obtained.

Because standards of mass (or "weights") are ordinarily calibrated and used on equal-arm balances (fig. 5), the effects of variations in the acceleration of gravity are self-eliminating and need not be taken into account. Two objects of equal mass will be affected in the same manner and by the same amount by any change in the value of the acceleration of gravity, and thus if they have the same weight, i. e., if they balance each other on an equal-arm balance, under one value of g, they will also balance each other under any other value of g.

On a spring balance, however, the weight of the body is not balanced against the weight of another body, but against the resistance of a spring. Therefore, using a very sensitive spring balance, the weight of a body would be found to change if the spring balance and the body were moved from one locality to another locality with a different acceleration of gravity. But a spring balance is usually used in one locality and is adjusted to indicate mass at that locality.

Another point that must be taken into account in the calibration and use of standards of mass is the buoyancy or lifting effect of the air. A body immersed in any fluid is buoyed up by a force equal to the weight of the displaced fluid. Two bodies of equal mass, if placed one on each pan of an equal-arm balance, will balance each other in a vacuum. A comparison in a vacuum against a known mass standard gives "true mass." If compared in air, however, they will not balance each other unless they are of equal volume. If of unequal volume, the

FIGURE 6.

Calibration of weights by NBS.

These two illustrations indicate extremes of weights routinely calibrated by NBS. The one on left shows the small weights (down to 0.05 mg) for use with microbalances. The illustration on right shows 1 of 2 standard test weight cars, owned and operated by NBS for calibrating and adjusting the master railway track scales of the Nation's railroads. The largest individual weight of these cars is 10 000 pounds. A total test load of 80 000 pounds is carried by each car.

larger body will displace the greater volume of air and will be buoyed up by a greater force than will the smaller body, and the larger body will appear to be lighter in weight than the smaller body. The greater the difference in volume, and the greater the density of the air in which the comparison weighing is made, the greater will be the apparent difference in weight. For that reason, in assigning a precise numerical value of apparent mass to a standard, it is necessary to base this value on definite values for the air density and the density of the mass standard of reference.

The corrections furnished by the National Bureau of Standards for the more precise mass standards are given both (a) on the basis of comparison in vacuum, and (b) on the basis of comparison against normal brass standards in air under standard conditions, with no correction applied for the buoyant effect of the air. Normal brass standards are defined as having a density of 8.4 grams per cubic centimeter at 0° C and a coefficient of cubical thermal expansion of 0.000 054 per deg C. Standard conditions are defined as air of 1.2 milligrams per cubic centimeter and temperature of 20° C. The corrections to be used with precise analytical weights are ordinarily given only in terms of apparent mass against normal brass standards.

c. Tests of Standards of Mass

Weights regularly used in ordinary trade and industry should be tested by State or local weights and measures officials. The National Bureau of Standards calibrates and certifies the values of weights submitted but it does not manufacture or sell weights. Information regarding the various classes of weights, the requirements for each class, the weight-calibration service of the Bureau and the regulations governing the submission of weights to NBS for test are contained in NBS Circular 547, section 1, Precision Laboratory Standards of Mass and Laboratory Weights, by T. W. Lashof and L. B. Macurdy (for sale by the Superintendent of Documents, U. S. Government Printing Office, Washington 25, D. C., at 25 cents a copy).

There is no physical standard of time corresponding to the standards of length and mass. Time is measured in terms of the motion of the earth; (a) on its axis, and (b) around the sun. The time it takes the earth to make a complete rotation on its axis is called a day, and the time it takes it to make a complete journey around the sun, as indicated by its position with reference to the stars, is called a year. The earth makes about 365 rotations on its axis (365.242 2, more exactly) while making a complete journey around the sun. In other words, there are almost exactly 365% solar days in a tropical or solar year. As it would be inconvenient and confusing to have the year, as used in everyday life, contain a fractional part of a day, fractional days are avoided by making the calendar year contain 365 days in ordinary years

average length of the calendar year as nearly as practicable equal to that of the tropical year, in order that calendar dates may not drift through the various seasons of the tropical year.

The earth, in its motion around the sun, does not move at a uniform speed, and the sun in its apparent motion does not move along the equator but along the ecliptic. Therefore the apparent solar days are not of exactly equal length. To overcome this difficulty time is measured in terms of the motion of a fictitious or "mean" sun, the position of which, at all times, is the same as would be the apparent position of the real sun if the earth moved on its axis and in its journey around the sun at a uniform rate. Ordinary clocks and watches are designed and regulated to indicate time in terms of the apparent motion of this fictitious or "mean sun." It is "mean noon" when this "mean sun" crosses the meridian, and the time between two successive crossings is a "mean solar day." The length of the mean solar day is equal to the average length of the apparent solar day.

In observing on the stars, the time generally used by astronomers is sidereal time. This is defined by the rotation of the earth with respect to the stars. A sidereal day is the interval between two successive passages of a star across a meridian. The sidereal day is subdivided into hours, minutes, and seconds, the hours being numbered from 1 to 24. The sidereal year is 365.256 36 solar days.

The mean solar day is divided into 24 hours, each hour into 60 minutes, and each minute into 60 seconds. Thus the mean solar second is 1/864 00 of a mean solar day, and this mean solar second is the unit in which short time intervals are measured and expressed.

The time at which the "mean sun" crosses the meridian at any point on the earth is known as "local mean noon." As it would be impracticable to use local mean time at each locality, the surface of the earth, by international agreement, has been divided into 24 standard time zones, each zone having a width of approximately 15 degrees of longitude. In each zone the time used is that corresponding to the meridian passing approximately through its center, and adjacent zones have a time difference of 1 hour.

The meridian passing through Greenwich, England, is taken as the standard, or prime meridian, and time throughout the world is reckoned with reference to the time at Greenwich. Each 15 degrees east or west from Greenwich corresponds to a time difference of 1 hour. There are a few exceptions to the above rule. East of Greenwich the time is faster, and west of Greenwich it is slower than at Greenwich.

The United States is divided into four time zones in which time is designated as Eastern, Central, Mountain, and Pacific. The time in these zones is slower than Greenwich time by 5,

NBS broadcasts various standard radiofrequency and time signals, day and night, from station WWV. These signals-accurate to 1 part n 50 000 000-are used by utilities to control 60-cycle generators, by manufacturers of electronic equipment to calibrate oscillators, by radio stations to keep their signals within assigned channels, by aircraft and ships, and by scientists and engineers in experimental work. WWV servicesincluding the standard musical pitch (440 cycles per second), precise time intervals (seconds), and time announcements in voice-can be picked up by shortwave receiver at 5, 10, 15, 20, and 25 megacycles per second.

Throughout the World, by R. E. Gould (for sale by the Superintendent of Documents, U. S. Government Printing Office, Washington 25, D. C., at 15 cents a copy).

The U. S. Naval Observatory has elaborate transit equipment with which it measures the time of rotation of the earth with respect to various heavenly bodies. The average time of rotation over a period of several days is used as a standard interval to which is compared the interval indicated by extremely accurate crystal clocks owned by the National Bureau of Standards. The time difference between these two intervals is published as a correction to the crystal clock time signals transmitted continuously by the National Bureau of Standards broadcasting station WWV (fig. 7) on 2.5, 5, 10, 15, 20, and 25 megacycles per second. The National Bureau of Standards time signal is accurate to one-millionth of a second over a 1-second interval, and accurate to 1 part in 50 million for intervals of 1 minute or longer. This signal provides an indispensable standard time interval for purposes of physical measurements, for quick and accurate calibration of timing devices, and for adjustment of very low frequency oscillators.

3.4. Standards of Capacity

Units of capacity, being derived units, in this country defined in terms of linear units, are not represented by fundamental standards. Laboratory standards have been constructed and are maintained at the National Bureau of Standards. These have validity only by calibration with reference either directly or indirectly with the linear standards. Similarly, standards of capacity have been made and distributed to the several States. Other standards of capacity have been verified by calibration for a wide variety of uses in science, technology, and commerce.

A portable cubic foot standard of volume (left) for field use in the fuel gas industry is being calibrated under laboratory conditions at NBS

by comparison with a laboratory standard immersion-type cubic-foot bottle (right center).