The operation of proximity sensing devices is reviewed. Basic concepts of electrical capacitance, potential distribution, electromagnetic field structure, and electrical bridge balancing are discussed. Capacitance changes and modeling for activation of proximity sensing devices are described. Mathematical expressions for the capacitance and potential distribution of sensor configurations are developed, and potential distributions as a function of geometry are derived. General configurations of equipotential lines are compared. Physical characteristics of commercially available coaxial cables are presented. Recommendations are made for tune up procedures, antenna considerations, electronics, and electromagnetic compatibility. The authors recommend that uniform test procedures and federal standards be developed for radio frequency proximity sensors.