Mercury has the lowest electrical conductivity of all the common metal elements, while selenium has the lowest thermal conductivity. Electrical conductivity is a measure of how well a substance can allow electricity to flow through it, and thermal conductivity is a similar measure of how well a metal allows heat to pass through.
Conductivity typically refers to electrical conductivity. It is determined by measuring the electrical resistivity of a material, which is measured in ohm-meters. The lower the resistivity, the higher the conductance of a material. The metals with the lowest electrical resistance and highest conductance are silver, copper and gold.