The use of ``phase modulated'' synchronized electronic oscillators is discussed in considerable detail, including the theoretical background as well as practical circuitry for measuring minor reactance changes (capacitance as low as 0.00004 pF or correspondingly low inductance). The proposed circuitry has some definite advantages over certain other schemes and has been used for some time in seismographic and other instrument design where remote sensing, taking advantage of reactance changes, is employed.