The glass harp consists of a set of wine glasses or goblets of different sizes, tuned by filling them with varying amounts of water. The musician produces sound by rubbing their moistened fingers around the rims of the glasses. This action causes the glass to vibrate and create a friction-induced 'stick-slip' phenomenon, resulting in a clear, pure sine-wave-like tone. Richard Pockrich is often credited with the first major performances on the glass harp in the 18th century. Unlike Benjamin Franklin's glass armonica, which mechanicalized the process, the glass harp remains a manual instrument. The pitch is determined by both the size of the glass and the volume of water; adding more water lowers the pitch because it increases the mass of the vibrating system. Today, professional glass harpists use high-quality crystal glasses that are sometimes ground to specific pitches so they don't require water at all.