The past two decades have seen a very dramatic rise in the production of megavoltage X-ray and electron accelerators for use in therapeutic medicine. Concomitant with this increase has been the need to develop accurate and reliable methods for measuring the deposition of radiation energy within the irradiated medium. Frequently, the ionometric methods used for assessing the absorbed dose in materials irradiated by X-ray beams generated at a few hundreds of kV have been extended to the megavoltage region, but, whilst the precision and reliability of such methods have generally been unaffected by this extension, the interpretation of such measurements and their relationship to the absorbed dose has become extremely complex. In this situation the technique of calorimetry offers the only direct and unambiguous approach for measuring the absorbed dose and this thesis deals with the design, construction and operation of such an instrument with the particular intention of calibrating a number of secondary dosimeters. The production of a stable temperature environment, which enables the small temperature changes encountered in calorimetry to be measured accurately, is discussed and it was concluded that a multi-jacket system, operated in the quasi-adiabatic mode, was the most suitable. The actual temperature measurements were made with a Wheatstone Bridge Network, using a thermistor transducer, and it is shown that if a particular criterion is adopted for the sensitivity of the system then the bridge can be so designed to maximise this sensitivity. The practical construction of a bridge system using a phase sensitive detector and capable of a temperature resolution of 2 x 10 5 K is also described.