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in other countries (see Standards). Verified “Parliamentary Copies” of the imperial standard are placed at the Royal Mint, with the Royal Society, at the Royal Observatory, and in the Westminster Palace.

The forms of the four primary standards representing the four units of extension and mass are shown in figs. 1 to 4.

A secondary standard measure for dry goods is the bushel of 1824, containing 8 imperial gallons, represented by a hollow bronze cylinder having a plane base, its internal diameter being double its depth.

The imperial standard measure of capacity is a hollow cylinder (fig. 5) made of brass, with a plane base, of equal height and diameter; which when filled to the brim, as determined by a plane glass disk, contains 10 ℔ weight of water at t =62° F.B.=30 in., weighed in air against brass weights.

Fig. 3.—National Standard Metre, 1897.

Iridio-platinum bar of Tresca section as shown at A. The two microscopic lines are engraved on the measuring axis of the bar at b, one near to each end of the bar. The standard metre (mètre à-traits) was supplemented by the delivery to Great Britain, in 1898, of an end standard metre (metre à bouts) also made of iridio-platinum, and also verified by the C.I.P.M. A comparison of the yard with the metre was made by the C.I.P.M. in 1896, and of the pound and and kilogram in 1883–1885 (see III. Commercial).

4. Atmospheric Pressure, and Materials.—In the verification of a precise standard of length there may be taken into account the influence of the variation of atmospheric pressure. Taking the range of the barometer in Great Britain from 28 to 31 in., giving a difference of 3 in. (76 millimetres), which denotes a variation of 103 grammes per square centimetre in the pressure of the atmosphere, the change caused thereby in the length of a standard of linear measurement would appear to be as follows:—

For the yard measure of the form shown in fig. 1 a difference of length equal to 0·000002 in. is caused by the variation of atmospheric pressure from 28 to 31 in. For the metre of the form shown in fig. 3 the difference in length for a variation of 76 mm. in the barometer would be 0·000048 mm. on the metre.

With reference to the materials of which standards of length are made, it appears that the Matthey alloy iridio-platinum (90% platinum, 10% iridium) is probably of all substances the least affected by time or circumstance, and of this costly alloy, therefore, a new copy of the imperial yard has been made. There appears, however, to be some objection to the use of iridio-platinum for weights, as, owing to its great density (∆=21·57), the slightest abrasion will make an appreciable difference in a weight, sometimes, therefore, quartz or rock-crystal is used; but to this also there is some objection, as owing to its low density (∆=2·65) there is a large exposed surface of the mass. For small standard weights platinum (∆=21·45) and aluminium (∆=2·67) are used, and also an alloy of palladium (60%) silver (40%) (∆=11·00).

For ordinary standards of length Guillaume’s alloy (invar) of nickel (35·7%) and steel (64·3%) is used, as it is a metal that can be highly polished, and is capable of receiving fine graduations. Its coefficient of linear expansion is only 0·0000008 for 1° C.^[1]

5. Electrical Standards.—Authoritative standards and instruments for the measurement of electricity, based on the fundamental units of the metric system, have been placed in the Electrical Laboratory of the Board of Trade.^[2] These include:

Current Measuring instruments.	$\scriptstyle {\left\{{\begin{matrix}\ \\\ \end{matrix}}\right.}$	The standard ampere, and sub-standards from 1 to 2500 amperes.
Potential measuring instruments.	$\scriptstyle {\left\{{\begin{matrix}\ \\\ \end{matrix}}\right.}$	The standard volt, and sub-standards for the measurement of pressure from 2·5 to 3000 volts.
Resistance measuring instruments.	$\scriptstyle {\left\{{\begin{matrix}\ \\\ \end{matrix}}\right.}$	The standard ohm, sub-standards up to 100,000 ohms, and below 1 ohm to 1/1000 ohm.

Fig. 4.—National Standard Kilogram, 1897.

6. Temperature.—In the measurement of temperature the Fahrenheit scale is still followed for imperial standards, and the Centigrade scale for metric standards. At the time of the construction of the imperial standards in 1844, Sheepshanks’s Fahrenheit thermometers were used; but it is difficult to say now what the true temperature then, of 62° F., may have been as compared with 62° F., or 16·667° C., of the present normal hydrogen scale. For metrological purposes the C.I.P.M. have adopted as a normal thermometric scale the Centigrade scale of the hydrogen thermometer, having for fixed points the temperature of pure melting ice (0°) and that of the vapour of boiling distilled water (100°), under a normal atmospheric pressure; hydrogen being taken under an initial manometric pressure of 1 metre, that is to say, at 1000/760=1·3158 times the normal atmospheric pressure. This latter is represented by the weight of a column of mercury 760 mm. in height; the specific gravity of mercury being now taken as 13·5950, after Volkmann and Marek, and at the normal intensity followed under this pressure. The value of this intensity is equal to that of the force of gravity at the Bureau International, Paris (at the level of the Bureau), divided by 1·000332; a co-efficient which allows for theoretical reduction to the latitude 45° and to the level of the sea. The length of the metre is independent of the thermometer so far that it has its length at a definite physical point, the temperature of melting ice (0° C.), but there is the practical difficulty that for ordinary purposes measurements cannot be always carried out at 0° C.

The International Geodetic Committee have adopted the metre as their unit of measurement. In geodetic measurements the dimensions of the triangles vary with the temperature of the earth, but these variations in the same region of the earth are smaller than the variations of the temperature of the air, less than 10° C. Adopting as a co-efficient of dilatation of the earth’s crust 0·000002, the variations of the distances are smaller than the errors of measurement (see Geodesy).

Fig. 5.—Present Imperial Standard Gallon, 1824.

7. Standardizing Institutions.—Besides the State departments dealing with weights and measures, there are other standardizing institutions of recent date. In Germany, e.g. there is at Charlottenburg (Berlin) a technical institute (Physikalisch - technische-Reichsanstalt) established under Dr. W. Förster in 1887, which undertakes researches with reference to physics and mechanics, particularly as applied to technical industries.^[3] In England a National Physical Laboratory (N.P.L.) has been established, based on the German institute, and has its principal laboratory at Bushey House, near

Hampton, Middlesex. Here is carried out the work of standardizing measuring instruments of various sorts in use

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1. Rapport du Yard. Dr. Benoit (1896).
2. Orders in Council (1894).
3. Wissenschaftliche Abhandlungen der Physikalischen Reichsanstalt. Band ii. (Berlin, 1900); Denkschrift betreffend die Tätigkeit der K. Norm.-Aichungs-Kommn. (1869–1900).