A method for measurement of soil thermal resistivity that is based on the theory that the rate of temperature rise of a line heat source is dependent upon the thermal constants of the medium in which it is placed is given. This information will enable the user to properly install and load underground cables. The aim is to provide sufficient information to enable the user to select useful commercial test equipment, or to manufacture equipment that is not readily available on the market, and to make meaningful resistivity measurements with this equipment, in the field or on soil samples in the laboratory. Designs for both laboratory and field thermal needles are described.