Page images
PDF
EPUB

ANNOUNCEMENT.

The Bulletin of the Bureau of Standards, of which this is the initial number, will embody the results of its investigations, researches, and other work which may be of importance to the scientific, technical, and manufacturing interests of the country. It will be issued at intervals as frequently as the number of papers ready for publication may require. Schedules of fees, regulations, and other matter pertaining to the testing work of the Bureau will be issued in the form of circulars of information.

S. W. STRATTON,

Director. WASHINGTON, D. C., October 15, 1904.

3

RECOMPARISON OF THE UNITED STATES PROTOTYPE

METER.

By Louis A. FISCHER.

Through the courtesy of Doctor Benoît, Director of the International Bureau of Weights and Measures, advantage was taken of a visit by the writer to Paris in October, 1903, to compare U. S. Prototype Meter No. 27 with the standards of the International Bureau.

U. S. METERS Nos. 27 AND 21.

U. S. Meter No. 27, like the prototypes of all the principal nations, and also like the international meter, is comiposed of 90 per cent platinum and 10 per cent iridium, with minute traces of other metals which compose less than 0.1 per cent of the total. It was intercompared at the International Bureau of Weights and Measures, in 1888, with the national prototypes above referred to and with the international meter; and shortly afterwards it was brought to this country by a special messenger, who certified that it had suffered no violent mechanical or temperature disturbance in transportation. Soon after its arrival in this country-or to be exact, on January 2, 1891—the standard was unpacked with considerable ceremony at the Executive Mansion in the presence of President of the United States, who accepted it as the national prototype meter.

It was then immediately repacked, sealed in its metal case, and taken to the Office of Standard Weights and Measures, in the custody of which it remained until the formation of the Bureau of Standards, on July 1, 1901, when it was transferred to the Bureau with the other apparatus belonging to the Office of Standard Weights and Measures. It remained packed and sealed in its case until a few weeks before it was taken to Europe, when it was compared with Meter No. 21, which is exactly similar to No. 27, except that the lines and surfaces are not as perfect as those of No. 27, on account of its having been frequently packed in shaved ice.

a For a full description of the prototype meter, its transportation and acceptance, see Appendix 18, U. S. Coast and Geodetic Survey Report for 1890.

COMPARISON OF No. 27 with No. 21.

This comparison was made solely to furnish a check on the length of No. 27 in case it should meet with accident in transporting it to and from Europe.

The comparisons were made on an improvised comparator installed in the subbasement of the Butler Building, in Washington, now occupied by the Bureau of Standards.

The comparator.The comparator, which is a temporary structure, differs essentially from those used at the International Bureau of Weights and Measures, and elsewhere, and in consequence the results at present obtained with it are not as concordant as the results obtained at the international bureau. For general purposes, however, it offers decided advantages, and if properly constructed, it is believed that it will give results just as concordant.

The essential features of the comparator are two brick piers, A A, which support the ends of a heavy iron I beam, B, to which are clamped two heavy iron brackets, CC. These brackets support the microscopes D D. The microscope supports may be clamped anywhere on the I beam, and hence the comparator may be used for comparing bars having any length from 0.1 meter to 1 meter. Where the microscopes are fixed to the piers it is only possible to compare bars of a definite length. By properly protecting the I beam and microscopes against temperature changes, or by making them of the 36 per cent nickelsteel alloy, it is believed that the distance between the microscopes will be as constant, or more so, than the distance between microscopes mounted on independent piers. In addition to the I beam and its two supporting piers, two intermediate piers, E E, support steel rails upon which the carriage F moves transversely to the I beam. Mounted upon this carriage is a wooden box G covered with sheet copper, inside of which is a heavy sheet brass box in which the two bars to be compared were placed. The object of this arrangement was to secure uniform temperature within the inner box. Since the coefficients of expansion of the two bars were almost identical, an accurate knowledge of the true temperature was not important, though it was important that both have the same temperature. The box rested upon three adjusting screws used to focus the bars under the microscopes.

To protect the I beam and the microscopes and clamps from the heat of the observers' bodies they were covered with sheet asbestos.

Thermometers. The temperatures of the two bars were determined by means of two Tonnelot thermometers previously studied at the International Bureau of Weights and Measures. One was placed upon each bar, the bulbs being placed in opposite directions, and the ther

mometer scales being read through small openings in the brass and wooden inclosing boxes. As before stated, the question of the actual temperature was not of importance, because the expansions of the two bars were so nearly equal that an error of a whole degree in reading

[merged small][merged small][merged small][merged small][ocr errors][merged small][merged small][merged small][merged small][merged small][ocr errors][ocr errors][merged small][merged small]

the temperature introduced an error in the final result of less than 0.01u. Should, however, the temperature of one bar differ from that of the other by 0.1° it would, if not corrected for, introduce an error of

« PreviousContinue »