In 1890 the account of a very accurate "Determination of the Specific Resistance of Mercury in Absolute Measure" by Lorenz's method was communicated by J. Viriamu Jones to the Royal Society, from which it followed that 106.307 centimetres was the required length of the mercury column which represented the ohm.* This number, in consequence of the great care that had been taken by Prof. Jones in arriving at it, may with safety be used to discriminate between the various lengths previously published, and it is seen that it is closely in accord with the result 106.29 obtained by Rowland, Kimball, and Duncan in 1884 by the use of this same method No. III., as well as with the value 106 31, which represents the result obtained in each of three separate investigations also carried out in 1884viz. by Mascart, De Nerville, and Benoît, using method No. IV., by the same experimenters using method No. V., and by Rowland and Kimball, also using method No. V. It also differs but little from Lord Rayleigh's result, 106-27, obtained by using method No. I., or from that deduced by Glazebrook, 106·28, from the employment of method No. V.

There is, then, a very strong reason for believing that the length of 106.3 centimetres is correct to the first four figures.

In December, 1890, the Board of Trade appointed the representatives of the Board of Trade, General

* The bobbin of the coil used in this investigation was made of brass, and yielded a little when it was being turned. This caused it to acquire a slightly elliptical shape with a difference in the lengths of the axes of about 1 part in 1,300. The value 106 307 given above for the ohm was decided on the assumption that the coil was truly circular, but, in a communication made to the Physical Society in May, 1896, Professor Jones has proved that the correction for the ellipticity is about 7 parts in 100,000. Hence, this determination of the specific resistance of mercury leads to the result that the length of the mercury column 1 square millimetre in cross-section, which has a resistance of 1 ohm at 0°C., is 106.300 centimetres (see page 599).

Post Office, Royal Society, British Association, and Institution of Electrical Engineers, whose names are given in a note to page 24, "to be a Committee to consider and report whether any, and if so, what, action should be taken by the Board of Trade under Section 6 of the Weights and Measures Act, 1889, with a view of causing new denominations of Standards for the measurement of electricity for use for trade to be made and duly verified."

The first report of this Committee was issued in July, 1891. It contained sixteen resolutions, of which the following were the most important:

"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.

"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 of a constant cross-sectional area of one square millimetre, and of a length of 106-3 centimetres, at the temperature of melting ice, may be adopted as one ohm.

"5. 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 9866 of the ohm.

"6. That a material standard, constructed in solid metal, and verified by comparison with the British Association unit, should be adopted as the standard ohm.

"9. That the standard of electrical current should be denominated the ampere, and should have the value one-tenth (0-1) on 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 ampere.

"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 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 ampere.

"14. That the electrical pressure at a temperature of 62°F. between the poles, or electrodes, of the voltaic cell known as Clark's cell, may be taken as not differencing from a pressure of 1433 volt, by more than an amount which will be determined by a sub-committee appointed to investigate the question, who will prepare a specification for the construction and use of the cell.

"16. That instruments constructed on the principle of Sir W. Thomson's Quadrant Electrometer used idiostatically, and, for high-pressure, 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."

Next followed the Specification (see § 6, pages 23-26) which was referred to in Resolution 10, and a Draft Order in Council proposed by the Committee for Her Majesty's signature.

In August, 1892, on the occasion of the meeting of the British Association at Edinburgh, there was a conference of its Committee on Electrical Standards with

Professor von Helmholtz, the director of the Imperial Physico-Technical Institute of Berlin, Dr. Guillaume, of the Bureau International des Poids et Mesures of France, and Professor Carhart, of the University of Michigan, U.S.A.

Professor von Helmholtz pointed out that in order to measure the bore of a narrow glass tube we must fill it with mercury and weigh it (see page 570), and therefore that it would be better to specify the weight than the cross-section of the column of mercury 106.3 centimetres in length that at 0°C. represented the ohm. He stated that from experiments carried out in his laboratories this weight was found to be 14 452 grammes, which, therefore, he had already recommended the German Government to adopt. He also mentioned that in the recommendations to his Government he had taken 15°C. as the standard temperature for the specification of the E.M.F. of the Clark's cell, and that at 15°C. the value was 1.434 volt.

The British Association Committee accordingly adopted resolutions in conformity with the preceding, and transmitted these resolutions to the Board of Trade. In consequence of this, the Committee of the Board of Trade, after further deliberation, issued a supplementary report in November, 1892, in which their former Resolution 4 was replaced by

"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,"

and their former Resolution 14 by

"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 1·434 volt by more than one part in one thousand."

Then followed the Specification referred to in Resolu

tion 14, which will be found in full in § 145, pages 467-469.

This substitution of the Centigrade for the Fahrenheit scale of temperature was only made after some discussion; for this supplementary report was the first document issued by the Board of Trade in which the Centigrade scale was officially recognised in Great Britain.

By 1892, then, both the English and the German. Governments were advised to adopt the resistance, at 0°C., of a column of mercury 106.3 centimetres long, of uniform cross-section, and weighing 14.4521 grammes, as the value of the ohm; whereas the French Government, some nine years before, had legalised as the ohm the resistance, at 0°C., of a column of mercury only 106 centimetres in length. Hence, before any material unit of resistance could receive international support, it was necessary to summon another international congress. The United States Government was, therefore, advised to utilise the occasion of the holding of the World's Fair at Chicago in 1893 by inviting the other Governments to co-operate with it in sending representatives to constitute a "Chamber of Delegates" for selecting the units of electrical measure. Five delegates were nominated by America, and the Governments of Great Britain, Germany, and France were each asked to nominate an equal number, while three, two, and in some instances one, were allotted to other countries.

The

Ten countries, as enumerated on page 173, were actually represented in the Chamber, and, after many sittings, it was agreed to adopt certain units, to each of which the name international was to be affixed. definitions of the international ampere and international volt were the same as those recommended by the Committee of the Board of Trade in the previous year, and the definition of the international ohm only differed from that of the Board of Trade ohm in that, while the latter had been defined as having "the value 1,000,000,000 in