Abstract: A double-tuning-fork (DTF) resonator having electrical circuitry operable, if either tine breaks, to develop a characteristic electrical circuit condition which is detected to produce an indication of failure. In one preferred embodiment, the circuitry includes a dual series circuit for energizing the electrodes on the DTF tines, with (1) a first series circuit passing down one tine to the remote end thereof, across the support segment at that end of the DTF, and back up the other tine, and (2) a second series circuit passing down the other tine to the remote end thereof, across said support segment at that end, and back up the first-mentioned tine. If either tine breaks, or if there is fracture of another part of the DTF carrying an electrode energizing lead, vibratory excitation of both tines ceases, and the output signal is caused to go to a level (e.g. zero) clearly indicating failure of the instrument.