Engineering measurements of internal friction in metals have been obtained from the decay characteristics of tuning forks of metals of interest. A damping program at Battelle Memorial Institute, conducted for the Office of Scientific Research and Development, called for such measurements at temperatures up to 1500 F, and it was necessary to develop suitable apparatus for obtaining accurate results at those elevated temperatures. A photoelectric method of measuring tine amplitude decay, using shutters on a vibrating fork to modulate light directed at a photocell, was developed. It proved to give very satisfactory results and had none of the mechanical difficulties in operation and uncertainties in results that were encountered at high temperatures when platinum-wire Sauereisen-cemented strain gages were used. It was necessary to reduce energy losses from the fork to its support by using a compliant coupling. This suspension worked so well that the lower limit of decrement measurements was probably determined by acoustic losses. It is believed that such losses added less than 0.00004 to the decrements of steel forks. This report includes details of construction of the equipment and reviews tests performed to establish its accuracy.

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