At very low frequencies, the output signal of electromagnetic transducers becomes too small to be useful for seismic sensing. One then uses active electronic transducers where a carrier signal, usually in the audio frequency range, is modulated by the motion of the seismic mass. The basic modulating device is an inductive or capacitive half-bridge. Inductive half-bridges are detuned by a movable magnetic core. They require no electric connections to the moving part and are environmentally robust; however their sensitivity appears to be limited by the granular nature of ferromagnetism. Capacitive half-bridges are realized as three-plate capacitors where either the central plate or the outer plates move with the seismic mass (Fig. 15). Their sensitivity is limited by the ratio between the electronic noise of the demodulator and the electrical field strength; it can be a hundred times better than that of the inductive type. The comprehensive paper by [Jones & Richards 1973] on the design of capacitive transducers still represents the state of the art in all essential aspects.