Page:EB1911 - Volume 06.djvu/881

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	$\scriptstyle {\left\{{\begin{matrix}\ \\\ \end{matrix}}\right.}$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 11/4% 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

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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.