In a conventional passive seismometer, the inertial force produced by a seismic ground motion deflects the mass from its equilibrium position, and the displacement or velocity of the mass is then converted into an electric signal. This principle of measurement is now used for short-period seismometers only. Long-period or broadband seismometers are built according to the `force-balance' principle. It means that the inertial force is compensated (or `balanced') with an electrically generated force so that the seismic mass moves as little as possible; of course some small motion is still required because otherwise the inertial force could not be observed. The feedback force is generated with an electromagnetic force transducer or `forcer' (Fig. 14). The electronic circuit is a servo loop (Fig. 16) like in an analog chart recorder. A servo loop is most efficient when it contains an integrator, in which case the offset of the mass is exactly nulled in the time average. Due to unavoidable delays in the feedback loop, force-balance systems have a limited bandwidth; however at frequencies where they are effective, they force the mass to move with the ground by generating a feedback force strictly proportional to ground acceleration. When the force is proportional to the current in the transducer, then the current, the voltage across the feedback resistor R, and the output voltage are all proportional to ground acceleration. We have thus converted the acceleration into an electric signal without depending on the precision of a mechanical suspension.
The response of a force-balance system is approximately inverse to the gain of the feedback path. It can easily be modified by giving the feedback path a frequency-dependent gain. For example, if we make the capacitor C large so that it determines the feedback current, then the gain of the feedback path increases linearly with frequency, and we have a system whose responsivity to acceleration is inverse to frequency and thus flat to velocity over a certain passband. We will look more closely at this option in section 4.3.