experimentalists for the resistivity of known metals differ to a considerable extent. I. CONDUCTION IN SOLIDS It is found convenient to express the resistivity of metals in two different ways: (1) We may state the resistivity of one cubic centimetre of the material in microhms or absolute units taken between opposed faces. This is called the _volume-resistivity_; (2) we may express the resistivity by stating the resistance in ohms offered by a wire of the material in question of uniform cross-section one metre in length, and one gramme in weight. This numerical measure of the resistivity is called the _mass-resistivity_. The mass-resistivity of a body is connected with its volume-resistivity and the density of the material in the following manner:--The mass-resistivity, expressed in microhms per metre-gramme, divided by 10 times the density is numerically equal to the volume-resistivity per centimetre-cube in absolute C.G.S. units. The mass-resistivity per metre-gramme can always be obtained by measuring the resistance and the mass of any wire of uniform cross-section of which the length is known, and if the density of the substance is then measured, the volume-resistivity can be immediately calculated. If R is the resistance in ohms of a wire of length l, uniform cross-section s, and density d, then taking [rho] for the volume-resistivity we have 10^9R = [rho]l/s; but lsd = M, where M is the mass of the wire. Hence 10^9R = [rho]dl²/M. If l = 100 and M = 1, then 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/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. TABLE I.--_Electric Mass-Resistivity of Various Metals at 0° C., or Resistance per Metre-gramme in International Ohms at 0° C._ (Matthiessen.)