Standards of Length

STANDARDS OF LENGTH The history of standards of length is one of varying ascend ency of three principal competing types. A length may be defined by the distance, under certain specified conditions, either between the two end surfaces of a material standard bar, or between two suitable marks engraved upon it. Alternatively reference may be made to some "natural" standard. The standard yards of Henry VII. and Elizabeth preserved in the Standards Department of the Board of Trade, are end standards, incisions marking subdivisions of the yard being secondary only. The Elizabethan yard was superseded by one defined by the distance between two small dots on gold plugs inserted in it. When this bar was legalized in 1824, it was provided that in the event of loss it should be replaced by reference to a "natural" standard, the length of the pendulum beating seconds in the latitude of London.

It so happened that within a very short period this bar was in fact destroyed by the fire in the Houses of Parliament in The commission charged with its replacement found, however, that it was impossible to reproduce the seconds pendulum with so high an accuracy as the length of the bar itself could be repro duced by means of direct comparison with other bars which had previously been compared with the lost standard. The new stand ard yard, which is legal at the present day, was thus restored.

The metre was originally intended to be the 1 o,000,000th part of the earth's meridional quadrant. But it was soon found that not only was the determination of this natural standard an ex tremely laborious undertaking, but the accuracy attainable was less than that possible in the comparison of material standards, ai.d the material Metre des Archives, a platinum end standard, became the accepted standard of reference for the metric system until superseded in 1889 by the present International Prototype Metre, a platinum iridium line standard (see WEIGHTS AND MEASURES).

Wave Lengths as Natural Standard.

Some later devel opments, however, have appreciably changed the situation. In the first place the experiments of Michelson, followed by those of Fabry and Perot (see LIGHT) have finally established a natural standard (the wave length of the red line in the spectrum of cad mium) which is reproducible with accuracy at least as great as that attainable in the comparisons of material standards, which is definitely free from the suspicion of possible secular variation in evitably attaching to all material standards, and by means of which the material standards necessarily employed in everyday practice can be verified in any part of the world without the risk of accident or damage involved in the periodical transport of material national reference standards to and from the international central laboratory for comparison with the prototype. Secondly.

such improvement has been effected during the last few years in the production of flat-ended standards that bars with accurately parallel ends of the quality of optical mirrors are now available, whose lengths can be more directly determined by the method of optical interference than is the case with line standards, and which are also more accurately comparable with each other.

The International Committee on Weights and Measures, at its meeting in 1923, decided in principle on a wave-length standard subject to experiment in the various national laboratories. Such experiments are proceeding and within the next decade a very important result may be achieved.

Use of Material Standards.

There is evidence that the im perial standard yard has probably shortened by about 0•0002 inch since it was originally constructed in 1844, but has remained un changed for the last 4o years. So far as is known, the interna tional prototype metre has remained unchanged since its verifica tion (1875-79). The evidence for this rests partly on subsequent recomparison with various national copies constructed in the same manner at the same time, and partly on two determinations, by different methods, at an interval of about 15 years, the first by Michelson and Benoit, and the second by Fabry and Perot, against the wave length of the red line of cadmium. These metre standards are all made of an alloy of 9o% platinum and io% iridium, regarded as the most satisfactory for ultimate standards. Its cost would be prohibitive for any other purpose.