Following a brief résumé of early attempts to vibrate a loaded beam in a vertical plane, which were unsuccessful, due either to bad wave form or the presence of excessive horizontal movement, a description is given of the final satisfactory method, which gave a controllable amplitude up to 0·002 inch at frequencies from 8 to 35 cycles/sec. with negligible horizontal movement and good wave form. The necessary force for vibrating the beam was obtained from resonant vibrations of an auxiliary mass-spring system tuned to the working frequency. The oscillations were maintained by a small crank and electric motor, and an important feature was the use of solid friction to give a true flat-topped resonance curve, thus permitting small changes of motor speed without variation of amplitude. The equivalent electrical circuit is given of the mechanical filter system used, and records are reproduced of wave forms obtained with different methods.