A portable ultrasonic radiometer, for use as a field survey instrument of ultrasonic therapy equipment, employs the radiation force principle and comprises a solenoid balance arm nulling device. One end of the balance arm includes a movable helical coil which moves inside of a fixed helical coil, and the other end supports an airbacked target. Substantially frictionless rotation of the balance arm is achieved through use of a jewel fulcrum. A reference position of the target is predetermined as the null point, and upon application of the ultrasonic power, deflection of the target occurs. Voltage across the moving coil is used to reposition the target at the null point, and the voltage required is an analogue of the total ultrasonic power. A digital read-out system with manual zeroing capability directly displays this power output.
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