When a solid body is being heated by the hotter fluid surrounding it (such as
a potato being baked in an oven), heat is first convected to the body and
subsequently conducted within the body. The Biot number is the ratio of the
internal resistance of a body to heat conduction to its external resistance to
heat convection. Therefore, a small Biot number represents small resistance
to heat conduction, and thus small temperature gradients within the body.