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 The Elizabethan yard was superseded by one defined by the distance between two small dots on gold plugs inserted in it.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Metre des Archives, a platinum end standard, became the accepted standard of reference for the metric system until superseded in 1889 by the present International Prototype Metre, a platinum iridium line standard (see WEIGHTS AND MEASURES).
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.