Page:EB1911 - Volume 06.djvu/880

If R is the resistance in ohms of a wire of length 𝑙, uniform cross-section s, and density d, then taking ρ for the volume-resistivity we have 10⁹R＝ρ𝑙/s; but 𝑙sd＝M, where M is the mass of the wire. Hence 10⁹R＝ρd𝑙²/M. If 𝑙＝100 and M＝1, then R＝ρ′＝resistivity in ohms per metre-gramme, and 10⁹ρ′＝10,000dρ, or ρ＝10⁵ρ′/d, and ρ′＝10,000MR/𝑙².

The following rules, therefore, are useful in connexion with these measurements. To obtain the mass-resistivity per metre-gramme of a substance in the form of a uniform metallic wire:—Multiply together 10,000 times the mass in grammes and the total resistance in ohms, and then divide by the square of the length in centimetres. Again, to obtain the volume-resistivity in C.G.S. units per centimetre-cube, the rule is to multiply the mass-resistivity in ohms by 100,000 and divide by the density. These rules, of course, apply only to wires of uniform cross-section. In the following Tables I., II. and III. are given the mass and volume resistivity of ordinary metals and certain alloys expressed in terms of the international ohm or the absolute C.G.S. unit of resistance, the values being calculated from the experiments of A. Matthiessen (1831–1870) between 1860 and 1865, and from later results obtained by J. A. Fleming and Sir James Dewar in 1893.

The data commonly used for calculating metallic resistivities were obtained by A. Matthiessen, and his results are set out in the Table II. which is taken from Cantor lectures given by Fleeming Jenkin in 1866 at or about the date when the researches were made. The figures given by Jenkin have, however, been reduced to international ohms and C.G.S. units by multiplying by (π/4)✕0·9866✕ 10⁵＝77,485.

Subsequently numerous determinations of the resistivity of various pure metals were made by Fleming and Dewar, whose results are set out in Table III.

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.