This document is a March 1949 Ad Interim Report on the development of a high-speed, low-sensitivity calorimeter at the Mound Laboratory. This calorimeter is a zero-compensating, thermocouple, twin type. The outside diameter of the aluminum tubing was machined for a slip fit into the glass test tube. Prior to assembling in the test tube, an aluminum plug was pressed in it midway from the ends. A groove was also milled on the outside down to the plug, and a copper-constantan thermocouple was secured in the groove after assembly. A single layer of aluminum foil was used to reduce radiation effects, and stray air currents were excluded with a cotton plug. The two arms of the calorimeter were placed in a three-liter beaker filled with water. The constantan leads of the two thermocouples were soldered together to make them compensating. The copper leads were connected to a L & N potentiometer, and a L & N galvanometer was also used. The potentiometer-galvanometer circuit was so connected that before each reading of the EMF of the thermocouples, the thermal EMF in the circuit could be accounted for. The galvanometer was then mounted on a heavy brass plate. The data show that when …