+--------------------------+------------------+------------------+ _Resistivity of Mercury._--The volume-resistivity of pure mercury is a very important electric constant, and since 1880 many of the most competent experimentalists have directed their attention to the determination of its value. The experimental process has usually been to fill a glass tube of known dimensions, having large cup-like extensions at the ends, with pure mercury, and determine the absolute resistance of this column of metal. For the practical details of this method the following references may be consulted:--"The Specific Resistance of Mercury," Lord Rayleigh and Mrs Sidgwick, _Phil. Trans._, 1883, part i. p. 173, and R. T. Glazebrook, _Phil. Mag._, 1885, p. 20; "On the Specific Resistance of Mercury," R. T. Glazebrook and T. C. Fitzpatrick, _Phil. Trans._, 1888, p. 179, or _Proc. Roy. Soc._, 1888, p. 44, or _Electrician_, 1888, 21, p. 538; "Recent Determinations of the Absolute Resistance of Mercury," R. T. Glazebrook, _Electrician_, 1890, 25, pp. 543 and 588. Also see J. V. Jones, "On the Determination of the Specific Resistance of Mercury in Absolute Measure," _Phil. Trans._, 1891, A, p. 2. Table IV. gives the values of the volume-resistivity of mercury as determined by various observers, the constant being expressed (a) in terms of the resistance in ohms of a column of mercury one millimetre in cross-section and 100 centimetres in length, taken at 0° C.; and (b) in terms of the length in centimetres of a column of mercury one square millimetre in cross-section taken at 0° C. The result of all the most careful determinations has been to show that the resistivity of pure mercury at 0° C. is about 94,070 C.G.S. electromagnetic units of resistance, and that a column of mercury 106.3 centimetres in length having a cross-sectional area of one square millimetre would have a resistance at 0° C. of one international ohm. These values have accordingly been accepted as the official and recognized values for the specific resistance of mercury, and the definition of the ohm. The table also states the methods which have been adopted by the different observers for obtaining the absolute value of the resistance of a known column of mercury, or of a resistance coil afterwards compared with a known column of mercury. A column of figures is added showing the value in fractions of an international ohm of the British Association Unit (B.A.U.), formerly supposed to represent the true ohm. The real value of the B.A.U. is now taken as .9866 of an international ohm. TABLE IV.--_Determinations of the Absolute Value of the Volume-Resistivity of Mercury and the Mercury Equivalent of the Ohm._