R = [rho]'= resistivity in ohms per metre-gramme, and 10^9[rho]'
= 10,000d[rho], or [rho] = 10^5[rho]'/d, and [rho]' = 10,000MR/l^2.

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.

TABLE I.--_Electric Mass-Resistivity of Various Metals at 0 deg. C.,
or Resistance per Metre-gramme in International Ohms at 0 deg. C._
(Matthiessen.)

+---------------------+---------------------+-------------+
| | Resistance at 0 deg.| |
| | C. in International | Approximate |
| Metal. | Ohms of a Wire | Temperature |
| | 1 Metre long and | Coefficient |
| | Weighing | near 20 |
| | 1 Gramme. | deg. C. |
+---------------------+---------------------+-------------+
|Silver (annealed) | .1523 | 0.00377 |
|Silver (hard-drawn) | .1657 | .. |
|Copper (annealed) | .1421 | 0.00388 |
|Copper (hard-drawn) | .1449 (Matthiessen's Standard) |
|Gold (annealed) | .4025 | 0.00365 |
|Gold (hard-drawn) | .4094 | .. |
|Aluminium (annealed) | .0757 | .. |
|Zinc (pressed) | .4013 | .. |
|Platinum (annealed) | 1.9337 | .. |
|Iron (annealed) | .765 | .. |
|Nickel (annealed) |