Details of design and construction of a simple and rugged thermocouple differential thermometer are presented. Gold +0.03 at. % iron wire is used, with niobium leads connected to a SQUID. It is demonstrated that at 1 K the response time is about 15 ms and that the temperature sensitivity of 10−7K with a 10‐Hz filter is Johnson‐noise limited. Additional qualities such as small size, little self‐heating, and high reproducibility make this device ideal for differential thermometry at low temperatures.