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The National Standards of Length and Mass, U. S. Prototype Meter 27 and U. S. Prototype Kilogram 20, are kept in this Standards Vault. Made of a stable alloy containing 90 percent of platinum and 10 percent of iridium, these standards are basic to all length and weight measurements in the United States. On daily display, the standards are safeguarded by a heavy alarm-protected glass door. View shows final inspection before standards were put on exhibition.

Preface

The subject of weights and measures is of universal interest. Millions of daily industrial operations and commercial transactions depend on a uniform and convenient system of weights and measures.

In its broadest sense the subject of weights and measures covers much more than the units used in the sale and purchase of commodities. Our high standard of living depends in large part on our ability to measure accurately everything from a loaded railroad car to the diameter of a submicroscopic particle. In fact, almost everyone makes daily use of an accepted system of measurement from the school child who studies arithmetic to the machinist who measures to a ten-thousandth of an inch, from the housewife who purchases a pound of butter to the manufacturer of automotive engines.

The National Bureau of Standards receives many requests for information on both the customary and metric systems of weights and measures. It is to serve this need-for teachers, students, and the general public-that this Circular has been prepared. It brings together much of the information that was previously available in separate mimeographed leaflets. For scientists. and industrialists who want more extensive information on the subject, the Bureau has published Units and Systems of Weights and Measures, Miscellaneous Publication 214.

A. V. ASTIN, Director.

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