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According to the Order in Council of August 23, 1894, by which certain fundamental electrical units were made legal— “The ohm which has the value 109 in terms of the centimetre, and the second of time, and is represented by 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 106.3 centims.,” is one of the units of electrical measurement on which “denomination of standards required for use in trade” is to be based. The establishment of the National Physical Laboratory rendered it possible to realize this unit in England. With this object, a number of selected tubes of “Verre dur” were obtained from M. Baudin, with the kind assistance of the officials of the Bureau International, while others of Jena 16'" glass were procured from Messrs. Schott and Co., Jena. The work, however, has necessarily occupied a long time. With the increasing accuracy of electrical measurements it appeared desirable to determine the constants of the tubes so that the probable error should not exceed 1 part in 100,000. Preliminary observations of various kinds were essential to secure success, while the work of organising the various departments of the Laboratory also tended to delay matters; hence it was not until the autumn of 1902 that the real start was made.

II. On the construction of some mercury standards of resistance, with a determination of the temperature coefficient of resistance of mercury