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. The resistivity of non-metallic conductors is in all cases higher than that of any pure metal. The resistivity of carbon, for instance, in the forms of charcoal or carbonized organic material and graphite, varies from 600 to 6000 microhms per cubic centimetre, as shown in Table VI.:-- TABLE VI.--_Electric Volume-Resistivity in Microhms per Centimetre-cube of Various Forms of Carbon at 15° C._ +-----------------------------------------+---------------+ | Substance. | Resistivity. | +-----------------------------------------+---------------+ | Arc lamp carbon rod | 8000 | | Jablochkoff candle carbon | 4000 | | Carré carbon | 3400 | | Carbonized bamboo | 6000 | | Carbonized parchmentized thread | 4000 to 5000 | | Ordinary carbon filament from glow-lamp | | | "treated" or flashed | 2400 to 2500 | | Deposited or secondary carbon | 600 to 900 | | Graphite | 400 to 500 | +-----------------------------------------+---------------+