# conductivity

**Electrical conductivity**, or specific conductance,
is the conductance of
a 1-meter cube of a substance, measured between opposite faces. Having
units of siemens per meter, conductivity
is the reciprocal of resistivity,
and expresses the substance's ability to conduct electricity.

The **equivalent conductivity**, Λ, of an electrolytic
solution is the conductivity of a solution divided by its concentration
in gram-equivalents (see equivalent
weight) per cubic meter, and is usually measured with an electrolytic
cell in a Wheatstone bridge.
The degree of ionic dissociation (*α*) was found by Arrhenius to be give by *α* = *Λ*/*Λ*_{0},
where *Λ*_{0} is the value of *Λ* extrapolated to zero concentration; this equation has since been shown
to apply only to weak electrolytes.

**Thermal conductivity** is a measure of the ability
of a substance to conduct heat (see thermal
conduction). For a block of material of cross section *A*,
the energy transferred per unit time *E*/*t*, between
faces a distance, *l*, apart is given by *E*/*t* = *λA*(*T*_{2} - *T*_{1})/*l*,
where *λ* is the conductivity and *T*_{2} and *T*_{1} are the temperatures of the faces. This assumes
that the opposite faces are parallel and that there is no heat loss
through the sides of the block. The SI unit of thermal conductivity
is therefore J s^{-1} m^{-1} K^{-1}.