The bar was made in 1844 to replace the Standard destroyed in 1834, when both Houses of Parliament were burned. The original Standard was the work of Bird, who produced it in 1760. In June, 1824, an Act had been passed legalising this Standard. It says:—
"The same Straight Line or Distance between the Centers of the said Two Points in the said Gold Studs in the said Brass Rod, the Brass being at the temperature of Sixty-two Degrees by Fahrenheit's Thermometer, shall be and is hereby denominated the 'Imperial Standard Yard.'"
To provide for accidents to the bar, the Act continues: "And whereas it is expedient that the said Standard Yard, if lost, destroyed, defaced, or otherwise injured, should be restored to the same Length by reference to some invariable natural Standard: And whereas it has been ascertained by the Commissioners appointed by His Majesty to inquire into the subject of Weights and Measures, that the Yard hereby declared to be the Imperial Standard Yard, when compared with a Pendulum vibrating Seconds of Mean Time in the Latitude of London in a Vacuum at the Level of the Sea, is in the proportion of Thirty-six Inches to Thirty-nine Inches and one thousand three hundred and ninety-three ten-thousandth Parts of an Inch."
The new bar was made, however, not by this method, but by comparing several copies of the original and striking their average length. Four accurate duplicates of the new standard were secured, one of which is kept in the Mint, one in the charge of the Royal Society, one at Westminster Palace, and the fourth at the Royal Observatory, Greenwich. In addition, forty copies were distributed among the various foreign governments, all of the same metal as the original.
The French metre has also been standardised, being equal to one ten-millionth part of a quadrant of the earth's meridian (i.e. of the distance from the Equator to either of the Poles), that is, to 39·370788 inches. Professor A. A. Michelson has shown that any standard of length may be restored by reference to the measurement of wave lengths of light, with an error not exceeding one ten-millionth part of the whole.
It might be asked "Why should standards of such great accuracy be required?" In rough work, such as carpentry, it does not, indeed, matter if measurements are the hundredth of an inch or so out. But when we have to deal with scientific instruments, telescopes, measuring machines, engines for dividing distances on a scale, or even with metal turning, the utmost accuracy becomes needful; and a number of instruments will be much more alike in all dimensions if compared individually with a common standard than if they were only compared with one another. Supposing, for instance, a bar of exact diameter is copied; the copy itself copied; and so on a dozen times; the last will probably vary considerably from the correct measurements.
Hence it became necessary to standardise the foot and the inch by accurate subdivisions of the yard. This was accomplished by Sir Joseph Whitworth, who in 1834 obtained two standard yards in the form of measure bars, and by the aid of microscopes transferred the distance between the engraved lines to a rectangular end-measure bar, i.e. one of which the end faces are exactly a yard apart.
He next constructed his famous machine which is capable of detecting length differences of one millionth of an inch. Two bars are advanced towards each other by screw gearing: one by a screw having twenty threads to the inch, and carrying a graduated hand-wheel with 250 divisions on its rim; the other by a similar screw, itself driven by a worm-screw, working on the rim, which carries 200 teeth. The worm-screw has a hand-wheel with a micrometer graduation into 250 divisions of its circumference. So that, if this be turned one division, the second screw is turned only 1 / 250 × 1 / 200 of a division, and the bar it drives advances only 1 / 20 × 1 / 200 × 1 / 250 = 1 / 1,000,000 of an inch. The screw at the other end of the machine (which in appearance somewhat resembles a metal lathe) is used for rapid adjustment only.