Standards. In the measurement of electricity the instruments Electrical are standardised by reference to verified standards, and standards and instruments are deposited with the Electrical Adviser of the Board of Trade, Major P. Cardew, R.E., at the Laboratory in Richmond Terrace, Whitehall, as follows: CURRENT MEASURING INSTRUMENTS. Sub-standards for measurement of currents as follows: Sub-standards for measurement of pressures as follows: Sub-standards up to 100,000 ohms, and below 1 ohm to Too ohm. The expression "meter" includes a large number of instruments which formerly were only regarded as scientific measuring instruments, but which now have come into common use, as volt-meters, am-meters, &c. The Weights and Measures Act, 1889 (s. 6), provides that the Board of Trade shall from time to time cause such new denominations for the measurement of electricity, temperature, pressure, or gravities, as appear to them to be required for use in trade to be made and duly verified; and in 1891 and 1892 a Committee on Standards for the Measurement of Electricity, appointed by the Board of Trade, made important recommendations (Reports, 1891 and 1892) to which effect was given by an Order in Council dated 23rd August 1894, the standards of the ohm, ampère, and volt, being legally defined for use in trade. To the Order is appended a specification dealing with the "Clark Cell." Recommendations in Report of Electrical Standards Committee, 1892. 1. That it is desirable that new denominations of standards for the measurement of electricity should be made and approved by Her Majesty in Council as Board of Trade standards. 2. That the magnitudes of these standards should be determined on the electro-magnetic system of measurement with reference to the centimetre as unit of length, the gramme as unit of mass, and the second as unit of time, and that by the terms centimetre and gramme are meant the standards of those denominations deposited with the Board of Trade. 3. That the standard of electrical resistance should be denominated the ohm, and should have the value 1,000,000,000 in terms of the centimetre and second. 4. That the resistance offered to an unvarying electric current by a column of mercury at the temperature of melting ice 14-4521 grammes in mass of a constant cross sectional area, and of a length of 106.3 centimetres may be adopted as one ohm. 5. That a material standard, constructed in solid metal, should be adopted as the standard ohm, and should from time to time be verified by comparison with a column of mercury of known dimensions. 6. That for the purpose of replacing the standard, if lost, destroyed, or damaged, and for ordinary use, a limited number of copies should be constructed, which should be periodically compared with the standard ohm. 7. That resistances constructed in solid metal should be adopted as Board of Trade standards for multiples and submultiples of the ohm. 8. That the value of the standard of resistance constructed by a committee of the British Association for the advancement of science in the years 1863 and 1864, and known as the British Association unit, may be taken as 0.9866 of the ohm. 9. That the standard of electrical current should be denominated the ampère, and should have the value of one-tenth (0·1) in terms of the centimetre, gramme, and second. 10. That an unvarying current which, when passed through a solution of nitrate of silver in water, in accordance with the specification attached to this Report, deposits silver at the rate of 0·001118 of a gramme per second, may be taken as a current of one ampère. 11. That an alternating current of one ampère shall mean a current such that the square root of the time average of the square of its strength at each instant in ampères is unity. 12. That instruments constructed on the principle of the balance, in which by the proper disposition of the conductors, forces of attraction and repulsion are produced, which depend upon the amount of current passing, and are balanced by known weights, should be adopted as the Board of Trade standards for the measurement of current whether unvarying or alternating. 13. That the standard of electrical pressure should be denominated the volt, being the pressure which, if steadily applied to a conductor whose resistance is one ohm, will produce a current of one ampère. 14. That the electrical pressure at a temperature of 15° Centigrade between the poles or electrodes of the voltaic cell known as Clark's cell, prepared in accordance with the specification attached to this report, may be taken as not differing from a pressure of 1434 volts, by more than one part in one thousand. 15. That an alternating pressure of one volt shall mean a pressure such that the square root of the time average of the square of its value at each instant in volts is unity. 16. That instruments constructed on the principle of Lord Kelvin's Quadrant Electrometer used idiostatically, and, for high pressures, instruments on the principle of the balance, electrostatic forces being balanced against a known weight, should be adopted as Board of Trade standards for the measurement of pressure, whether unvarying or alternating. tempera In connection with measuring instruments thermo- Measuremeters are used, and in the measurement of temperature ment of for weights and measures purposes, the Fahrenheit scale ture. (32° to 212° F.) is legally adopted; for instance, the Imperial yard has its true length at 62° F.; the gallon measure contains its true weight of water at 62° F.; and the unit of measure in the sale of gas is the cubic foot at 62° F., &c. All metric weights and measures are, however, referred to the Centigrade scale (0° to 100° C.) and the Centigrade scale is also used in connection with electrical standards. There is no legal standard of the Fahrenheit thermometer, but three representative thermometers, originally verified by Sheepshanks, and one presented by the Kew Committee of the Royal Society in 1869 ; and also a standard centigrade thermometer; are deposited at the Standards Office. The relation of the Fahrenheit thermometer to the Centigrade scale in terms of the hydrogen thermometer has been thereby so far determined by the Comité International des Poids et Mesures, 62° F. being taken as equivalent to 16-667 C. The Centigrade scale, has had its degree carefully defined; a "normal degree" is a degree of the Centigrade thermometer when the point 100° C. corresponds to the temperature of pure boiling water under the pressure of a column of mercury (A = 13.59593) at the height of 760 millimetres at 0° C., in latitude 45° and at the level of the sea; the zero point of the scale being at the temperature of melting ice. O One of the first attempts made in this country to establish accurate Fahrenheit thermometers was probably that made by the Rev. R. Sheepshanks, F.R.S., in 1847, before which time it would appear that there was no workman in London who could be trusted to make a good thermometer. Sheepshanks' standards were followed by those of Balfour Stewart (Kew), and of Clarke (Comparisons of Standards, 1866). The old form of mercurial thermometer, having spherical bulbs (like Sheepshanks'), or containing a large mercurial column, has been now superseded for standards purposes by the new form of thermometer adopted by the Comité. The above standard Centigrade thermometer has a cylindrical bulb (1·7 × 0·2 inch) with a very fine column of mercury; it was made in 1890, and was originally verified by the Comité in relation to the hydrogen thermometer. Besides this standard there are two other similar standards which accompany the metres prototypes. This thermometer is about 702 millimetres long; the length of a normal degree being 5.3 mm.; and it is divided throughout into 0°1 C. from 4° to 103°. Its error at the boiling point was 0°·02 C. and at the zero point 0° 07 C. when placed in a horizontal position, and there is also a small correction for external atmospheric pressure; and for reduction to sea level, and to the normal latitude of 45° (vide Guillaume's Thermometrie de Précision, p. 30). When placed in a vertical position the "co-efficient of pressure," or correction for the internal pressure of the mercury for one millimetre of mercury is only 0.0001. This thermometer is made of hard glass (Tonnelot) in which the zero point is practically constant, showing on analysis 71 5 silica, 14.5 lime, 11.2 soda, 1·3 alumina, 0·7 sulphuric acid, 0·3 potash, and per-oxide of iron, 0·3. Mr. Sheepshanks has given an account in the Monthly Notices of the Royal Astronomical Society for June 1852 (reprinted by George Barclay, Leicester Square, 1852), of his proposed method of constructing standard thermometers. He thought that he could produce instruments which, with certain precautions, would show a thirtieth or even a fiftieth of a degree Fahrenheit, but he found that, after determining the boiling point, the freezing point of a thermometer would fall from one-third to one-fourth of a degree. scales. The International Meteorological Tables (Paris, 1890), Barometer published in conformity with the resolution of a Congress at Rome (1879), state, with reference to the comparison of the different barometrical scales, as follows (Chap. 4, p. B. 23): "The correct reading of the height of the barometer supposes a two fold correction of temperature to bring on the one hand the divisions of the scale to the normal temperature of the standard of length, and on the other, the mercury in the barometer to a fixed temperature, viz., the melting point of ice. Thus in France, the mercury and the scale are set to 0° C.,whilst in England the mercury is set to 32° F., and the scale to 62° F. which is the normal |