A set of graphical plots used to determine the time taken for thermal penetration by heat conduction into a solid body by heating or cooling at its surface. The plots are prepared for standard geometric shapes such as slabs, cylinders, and spheres with the Fourier number on the x-axis and a dimensionless temperature on the y-axis. The lines represent the reciprocal of the Biot number. They are named after M. P. Heisler, who compiled them in 1947.