onductivity. - The electrical conductivity of a substance is the electrical conducting power of a unit length per unit of cross-sectional area. The electrical resistance of metals or alloys is increased by decreasing the size of the cry - stals and, therefore, increasing the number of crystal bound - aries. In general, all metals increase in resistivity with increase in impurities. The resistivity of metals is also increased in most cases by an increase in temperature.
Heat Conductivity. - Heat conductivity is measured as the heatconducting ability of a unit length or thickness of a sub - stance per unit of cross-sectional area.
Magnetism. - Magnetism is measured as the magnetic for - ce exerted by a unit volume of a substance under standard ma - gnetizing force. Iron, cobalt and nickel are the only metals possessing considerable magnetism at room temperature, and they become non-magnetic when heated to a certain tempera - ture. Strong permanent magnets have been made chiefly of one of several compositions of steel, but in recent years a number of magnet alloys of much greater magnetism, able to exert forces many times their own weights, have been de - veloped.
Density and Porosity. - Porosity, the quality of containing pores is lack of denseness. Density, on the other hand, denotes weight per unit of volume. The distinction will be manifest from the fact that some heavy metals, like grey cast ironware porous enough to leak under heavy hydraulic pressures, whe - reas some lightweight metals, like aluminium, are dense and compact.
Most metals expand on heating and contract on cooling.
Colour. - Most of the metals are silvery white or grey in colour. Copper is the only red metal, and gold the only yellow one, although a number of copper-base a
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