Thermal flow will take place between substances in contact, or in close proximity, when the temperature levels differ. The transference of thermal energy always flows from the hotter substance to the cooler substance, regardless of the quantities of thermal energy held by each. The flow of thermal energy will continue in this direction, until a temperature difference no longer exists. The rate of thermal flow will be determined by the extent of the difference between the levels of temperature in the two substances. The rate of transfer of thermal energy is a characteristic physical property of each material termed thermal conductivity, a factor often requiring analysis when setting up welding parameters. Reference: George E. Linnert, Welding Metallurgy, Vol. 1, 4th Edition, Miami, Florida: American Welding Society, 1994.