26 MEASURING AND SCIENTIFIC APPLIANCES-NAMELY, PHASE TRACKING RECEIVERS, FREQUENCY DIFFERENCE METERS, OSCILLATORS, OSCILLATOR CIRCUITS, OSCILLATOR AND PHASE COMPARATOR CIRCUITS, ELECTRICAL ENERGY DISCRIMINATORS, ELECTRICAL COUNTERS, MAGNETIC COUNTERS, AND FREQUENCY DIVIDERS, AND CALIBRATIONAL, STANDARDIZATIONAL AND NAVIGATIONAL INSTRUMENTS INVOLVING SUCH COMPONENTS