A platinum resistance thermometer is described capable of reading temperatures in the range +37 degrees C to -200 degrees C with an accuracy of +or-0.1 degrees C. The sensor is a standard industrial platinum element, and the measuring bridge is a simple d.c. circuit employing a robust null-point detector. A calibration procedure involving the use of four convenient fixed points is described, and a simple method of interpolation is given. The performance of the instrument during 12 months of continuous laboratory use is described.