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The paper describes an instrument termed the `vibration wattmeterÂ¿ which consists of a cavity in which is suspended a thin metal rod mounted on a fine glass spindle with a quartz fibre suspension. The rod is free to execute torsional oscillations in the horizontal plane. The oscillating system receives periodic impulses from the interaction between the metal rod and the electromagnetic field within the cavity, and by the automatic switching of the microwave source a steady amplitude of mechanical oscillations can be maintained. The amplitude of these oscillations is measured by means of a small mirror attached to the spindle and a lamp-and-scale arrangement. The relationship between the power delivered to the cavity and the amplitude of the mechanical oscillations can be found theoretically, and with knowledge of the specific couple of the quartz fibre suspension and of the damping factor of the mechanical system, measurements of power can be made. The instrument measures microwave power with an error estimated as not exceeding Â±2%. The measurement is absolute, since the calibration depends only on measurements of mass, length and time.