Ultrasound radiation force balance. This equipment is used to calibrate high intensity focused ultrasound (HIFU) transducers. It comprises a target suspended in a water bath, whose weight changes when ultrasonic energy is produced by the transducer. The force exerted on the target by the transducer is detected by a microbalance. The total ultrasonic power of the transducer is calculated from the measured force multiplied by the speed of the ultrasound passing through the water bath. The power can then be adjusted if it is found to be too high or too low. Photographed at the National Physical Laboratory in Teddington, UK.