These include the develop¬ ment and maintenance of the national standards of measurement and the provision of means and methods for making measurements consistent with these standards; the determination of physical constants and properties of materials; the development of methods and instruments for testing materials, devices, and structures; advisory services to Government Agencies on scientific and technical problems; invention and development of devices to serve special needs of the Government; and the development of standard practices, codes, and specifications. The work includes basic and applied research, development, engineering, instrumentation, testing, evaluation, calibration services, and various consultation and information services. A major portion of the Bureau's work is performed for other Government Agencies, particularly the Department of Defense and the Atomic Energy Commission. The scope of activities is suggested by the listing of divisions and sections on the inside of the back cover. their gratitude to members of these two groups who have read the manuscript and have given them the benefit of their criticisms.

Constructional d etails of t he National Bureau of Standards' new free-a ir chamber for the m eas urement of 10 to 60 kV x rays in roentgens are given . The res ults of t he comparisons of t he new cham ber with t he National Burea u of Standards' "low" energy standard in t heir overlappin g ran ge a re included. The two standa rd cha mbers, on t he a verage, agreed to w ithin 0.3 percen t. The maximum ull cE:rtainties in t he correction factors used for t hc co mpari son m easu rements are exam ined a nd t heir sum is co mpared with t he res ults obtained for t he chamber co mparisons. An estimate is also made of t he maximum uncertainty, a bout 1.3 percent, of an expos ure rate d etermination to be expected when using t he 10 to 60 kV cha mber to measure 10 to 15 kV x rays.

Information is provided on the correction factor required for the determination of t hc dose in roentgens of 250-to 500-kilovolt X-rays from measurement with a free-air ioni zation chamber. Sp ecial emphasis is placed 011 the determinatio n of eac ll of t he factors so t hat an over-all accuracy of 0.5 percent can be obtained.

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