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SOLIDS]
857
CONDUCTION, ELECTRIC


Table IV.Determinations of the Absolute Value of the Volume-Resistivity of
Mercury and the Mercury Equivalent of the Ohm.

Observer. Date. Method. Value of
B.A.U. in
Ohms.
Value of 100
 Centimetres of 
Mercury in
Ohms.
Value of Ohm
 in Centimetres 
of Mercury.
Lord Rayleigh 1882 Rotating coil .98651 .94133 106.31
Lord Rayleigh 1883 Lorenz method .98677 .. 106.27
G. Wiedemann 1884 Rotation through 180° .. .. 106.19
E. E. N. Mascart 1884 Induced current .98611 .94096 106.33
H. A. Rowland 1887 Mean of several methods .98644 .94071 106.32
F. Kohlrausch 1887 Damping of magnets .98660 .94061 106.32
R. T. Glazebrook 1882
1888
Induced currents .98665 .94074 106.29
Wuilleumeier 1890   .98686 .94077 106.31
Duncan and Wilkes 1890 Lorenz .98634 .94067 106.34
J. V. Jones 1891 Lorenz .. .94067 106.31
    Mean value .98653    
Streker 1885 An absolute determination   .94056 106.32
Hutchinson 1888 of resistance was not   .94074 106.30
E. Salvioni 1890 made. The value .98656   .94054 106.33
E. Salvioni .. value .98656 has been used   .94076 106.30
    Mean value .94076 106.31
H. F. Weber 1884 Induced current Absolute measurements 105.37
H. F. Weber .. Rotating coil  compared with German 106.16
A. Roiti 1884 Mean effect of induced current  silver wire coils issued by 105.89
F. Himstedt 1885    Siemens and Streker 105.98
F. E. Dorn 1889 Damping of a magnet   106.24
Wild 1883 Damping of a magnet   106.03
L. V. Lorenz 1885 Lorenz method   105.93

For a critical discussion of the methods which have been adopted in the absolute determination of the resistivity of mercury, and the value of the British Association unit of resistance, the reader may be referred to the British Association Reports for 1890 and 1892 (Report of Electrical Standards Committee), and to the Electrician, 25, p. 456, and 29, p. 462. A discussion of the relative value of the results obtained between 1882 and 1890 was given by R. T. Glazebrook in a paper presented to the British Association at Leeds, 1890.

Resistivity of Copper.—In connexion with electro-technical work the determination of the conductivity or resistivity values of annealed and hard-drawn copper wire at standard temperatures is a very important matter. Matthiessen devoted considerable attention to this subject between the years 1860 and 1864 (see Phil. Trans., 1860, p. 150), and since that time much additional work has been carried out. Matthiessen’s value, known as Matthiessen’s Standard, for the mass-resistivity of pure hard-drawn copper wire, is the resistance of a wire of pure hard-drawn copper one metre long and weighing one gramme, and this is equal to 0.14493 international ohms at 0° C. For many purposes it is more convenient to express temperature in Fahrenheit degrees, and the recommendation of the 1899 committee on copper conductors[1] is as follows:—“Matthiessen’s standard for hard-drawn conductivity commercial copper shall be considered to be the resistance of a wire of pure hard-drawn copper one metre long, weighing one gramme which at 60° F. is 0.153858 international ohms.” Matthiessen also measured the mass-resistivity of annealed copper, and found that its conductivity is greater than that of hard-drawn copper by about 2.25% to 2.5% As annealed copper may vary considerably in its state of annealing, and is always somewhat hardened by bending and winding, it is found in practice that the resistivity of commercial annealed copper is about 1¼% less than that of hard-drawn copper. The standard now accepted for such copper, on the recommendation of the 1899 Committee, is a wire of pure annealed copper one metre long, weighing one gramme, whose resistance at 0° C. is 0.1421 international ohms, or at 60° F., 0.150822 international ohms. The specific gravity of copper varies from about 8.89 to 8.95, and the standard value accepted for high conductivity commercial copper is 8.912, corresponding to a weight of 555 lb per cubic foot at 60° F. Hence the volume-resistivity of pure annealed copper at 0° C. is 1.594 microhms per c.c., or 1594 C.G.S. units, and that of pure hard-drawn copper at 0° C. is 1.626 microhms per c.c., or 1626 C.G.S. units. Since Matthiessen’s researches, the most careful scientific investigation on the conductivity of copper is that of T. C. Fitzpatrick, carried out in 1890. (Brit. Assoc. Report, 1890, Appendix 3, p. 120.) Fitzpatrick confirmed Matthiessen’s chief result, and obtained values for the resistivity of hard-drawn copper which, when corrected for temperature variation, are in entire agreement with those of Matthiessen at the same temperature.

The volume resistivity of alloys is, generally speaking, much higher than that of pure metals. Table V. shows the volume resistivity at 0° C. of a number of well-known alloys, with their chemical composition.


Table V.Volume-Resistivity of Alloys of known Composition at 0° C. in C.G.S.
Units per Centimetre-cube. Mean Temperature Coefficients taken at 15° C.
(Fleming and Dewar.)
Alloys. Resistivity
at 0° C.
Temperature
Coefficient
at 15° C.
Composition in per cents.
Platinum-silver 31,582  .000243 Pt 33%, Ag 66%
Platinum-iridium 30,896  .000822 Pt 80%, Ir 20%
Platinum-rhodium 21,142  .00143 Pt 90%, Rd 10%
Gold-silver 6,280  .00124 Au 90%, Ag 10%
Manganese-steel 67,148  .00127 Mn 12%, Fe 78%
Nickel-steel 29,452  .00201 Ni 4.35%, remaining percentage
       chiefly iron, but uncertain
German silver 29,982  .000273 Cu5Zn3Ni2
Platinoid[2] 41,731  .00031  
Manganin 46,678  .0000 Cu 84%, Mn 12%, Ni 4%
Aluminium-silver 4,641  .00238 Al 94%, Ag 6%
Aluminium-copper 2,904  .00381 Al 94%, Cu 6%
Copper-aluminium 8,847  .000897 Cu 97%, Al 3%
Copper-nickel-aluminium 14,912  .000643 Cu 87%, Ni 6.5%, Al 6.5%
Titanium-aluminium 3,887  .00290  
Generally speaking, an alloy having high resistivity has poor mechanical qualities, that is to say, its tensile strength and ductility are small. It is possible to form alloys having a resistivity as high as 100 microhms per cubic centimetre; but, on the other hand, the value of an alloy for electro-technical purposes is judged not merely by its resistivity, but also by the degree to which its resistivity varies with temperature, and by its capability of being easily drawn into fine wire of not very small tensile strength. Some pure metals when alloyed with a small proportion of another metal do not suffer much
  1. In 1899 a committee was formed of representatives from eight of the leading manufacturers of insulated copper cables with delegates from the Post Office and Institution of Electrical Engineers, to consider the question of the values to be assigned to the resistivity of hard-drawn and annealed copper. The sittings of the committee were held in London, the secretary being A. H. Howard. The values given in the above paragraphs are in accordance with the decision of this committee, and its recommendations have been accepted by the General Post Office and the leading manufacturers of insulated copper wire and cables.
  2. Platinoid is an alloy introduced by Martino, said to be similar in composition to German silver, but with a little tungsten added. It varies a good deal in composition according to manufacture, and the resistivity of different specimens is not identical. Its electric properties were first made known by J. T. Bottomley, in a paper read at the Royal Society, May 5, 1885.