A system has been designed and constructed to measure thermal conductivity by the series comparative method in the temperature range 4 to 300K. A sample temperature of 77K was reached when the cylindrical sample chamber 3-1/4 inches in diameter and 8-3/4 inches long was evacuated to a pressure of 0.000003 Torr and submerged in a bath of liquid nitrogen held in a 5-1/2 liter stainlesssteel dewar. Temperatures as low as 4.2K could be obtained if the sample chamber were submerged in liquid helium. Temperatures between that of the liquid bath and room temperature were obtained by electrical heating. A simple Wheatstone bridge circuit, one arm of which was a copper resistance thermometer made from 140 ohms of B and S No. 36 copper wire, served to regulate automatically the electrical heating so as to give a drift in sample temperature of less than 0.002 deg/min. Measurements were attempted from 77K to 300K on a sample of n-type single-crystal silicon with an impurity concentration of 5X10 to the 15th power cu cm. Armco iron was used as a standard. Temperatures were measured with copper vs constantan thermocouples. The thermal conductivity of the sample was found to range from 8.7 watt/cm-degK at 98K to 1.8 watt/cm-degK at 255K. These values agree with those of Carruthers et al. Within 7% and of Glassbrenner and Slack within 10%. (Author).