A new generation of electrostatic high impedance control elements is now becoming available to supplement the current range of electromagnetic, low impedance devices. One of the most representative is tho crystal motor group, e.g. two-phase/one-bridge, two-phase/two-bridge motors operating in piezo-resonant mode. The basic principle of operation is the converse piezoelectric effect, which makes use of the mechanical strain produced in the crystal structure by a controlled electrostatic field.