The rate of thermal conduction through matter is proportional to the thermal
gradient:
(This formula was misprinted in a previous lecture. The time derivative
of Q was ommitted)
The constant of proportionality, 
is called the thermal conductivity
and is a materials parameter. We previously discussed thermal conduction as a means
of heat flow.
The thermal resistance is characteristic not only of the material but also of
the geometry involved, specifically the total length which supports the temperature
difference.
The thermal resistance is large for thick matter with low thermal conductivity.
In terms of the thermal resistance we can calculate the rate of heat flow
from
I want to draw your attention to the important formulae for calculating the effective
thermal resistance for serial and parallel combinations of thermal resistors.
To exercise the formulae we solve problems which give examples of
both a series and a parallel connection of thermal resistors.