A large number of beautiful patterns formed on the surface of a piezo-electric quartz plate by the glow discharge at low pressures of gas were observed at various radio frequencies of the exciting oscillator. They are caused by different modes of vibration of the plate of which there seems to be a great number. Evidence in support of the theory that the bright spots in the pattern occur at the antinodes of standing waves in the plate is presented from which it appears that the value of Young’s modulus is the same for directions at angles of from 0 to 78 degrees from the optic axis. These patterns explain the multitude of frequencies at which a piezo-electric quartz plate will vibrate observed by Cady and others.
Water vapor was found to condense on the crystal in the configuration of the pattern corresponding to the last mode of vibration of the crystal and a thin spark was produced about 13 mm long between the electrodes and over the surface of the vibrating crystal at a pressure of 5 mm.
A. M. SKELLETT, "A VISUAL METHOD FOR STUDYING MODES OF VIBRATION OF QUARTZ PLATES," J. Opt. Soc. Am. 17, 308-310 (1928)