An improved instrument has been designed and constructed for the measurement of the linear decomposition and sublimation rates of solid substances. This instrument, which is provided with automatic recording devices, eliminates many of the shortcomings of the earlier prototypes and makes possible the determination of linear rates with a precision of 1%. Important new features include (1) an integral heating element and thermocouple assembly; (2) stainless‐steel and Plexiglas construction throughout; (3) heavy, solid‐copper electrical‐conductor supports for the heating element; and (4) a precision position‐sensing potentiometer. The use of cut strands of Plexiglas 55 is recommended for periodic standardization and calibration of the instrument.