Metal.Resistance at 0° C.
per Centimetre-cube
in C.G.S. Units.		Mean Temperature
Coefficient between
0° C. and 100° C.
Silver (electrolytic and well annealed)[6]1,468    0.00400
Copper (electrolytic and well annealed)[6]1,561    0.00428
Gold (annealed)2,197    0.00377
Aluminium (annealed)2,665    0.00435
Magnesium (pressed)4,355    0.00381
Zinc5,751    0.00406
Nickel (electrolytic)[6]6,935    0.00618
Iron (annealed)9,065    0.00625
Cadmium10,023    0.00419
Palladium10,219    0.00354
Platinum (annealed)10,917    0.003669
Tin (pressed)13,048    0.00440
Thallium (pressed)17,633    0.00398
Lead (pressed)20,380    0.00411
Bismuth (electrolytic)[7]110,000    0.00433

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 1882/8 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[8] 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-silver31,582 .000243Pt 33%, Ag 66%
Platinum-iridium30,896 .000822Pt 80%, Ir 20%
Platinum-rhodium21,142 .00143Pt 90%, Rd 10%
Gold-silver6,280 .00124Au 90%, Ag 10%
Manganese-steel67,148 .00127Mn 12%, Fe 78%
Nickel-steel29,452 .00201Ni 4.35%, remaining percentage
 chiefly iron, but uncertain
German silver29,982 .000273Cu5Zn3Ni2
Platinoid[9]41,731 .00031
Manganin46,678 .0000Cu 84%, Mn 12%, Ni 4%
Aluminium-silver4,641 .00238Al 94%, Ag 6%
Aluminium-copper2,904 .00381Al 94%, Cu 6%
Copper-aluminium8,847 .000897Cu 97%, Al 3%
Copper-nickel-aluminium14,912 .000643Cu 87%, Ni 6.5%, Al 6.5%
Titanium-aluminium3,887 .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 change in resistivity, but in other cases the resultant alloy has a much higher resistivity. Thus an alloy of pure copper with 3% of aluminium has a resistivity about 5½ times that of copper; but if pure aluminium is alloyed with 6% of copper, the resistivity of the product is not more than 20% greater than that of pure aluminium. The presence of a very small proportion of a non-metallic element in a metallic mass, such as oxygen, sulphur or phosphorus, has a very great effect in increasing the resistivity. Certain metallic elements also have the same power; thus platinoid has a resistivity 30% greater than German silver, though it differs from it merely in containing a trace of tungsten.