The paper is concerned with the measurement and application of radiation-pressure forces, operating at millimetre wavelengths on low-loss dielectric situated in the electromagnetic field, when there is a gradient of stored energy in the material. To strengthen the applied field, the dielectric is placed in a resonant cavity, and observations of the mechanical force show it to be substantially in accordance with theory. A novel experimental wattmeter operated by such a force is described, and a further development of it, which uses a twin system of cavities giving differential forces arising from forward- and backward-travelling waves in the main power-carrying guide, is shown to yield a practical instrument for absolute power measurements.