Simple electrodynamic seismometers (geophones) have no calibration coil. The calibration current must then be sent through the signal coil. There it produces an ohmic voltage in addition to the output signal generated by the motion of the mass. The undesired voltage can be compensated in a bridge circuit [Willmore 1959]; the bridge is zeroed with the seismic mass locked or the geophone tilted so that the mass rests at a stop. When the calibration current and the output voltage are digitally recorded, it is more convenient to use only a half-bridge (Fig. 22) and to compensate the ohmic voltage numerically. The program CALEX decribed below has provisions to do this automatically.
Geophones whose seismic mass moves along a straight line require no mechanical calibration when the size of the mass is known. The electromagnetic part of the numerical damping is inversely proportional to the total damping resistance (Eq. 36); the factor of proportionality is , so the generator constant E can be calculated from electrical calibrations with different resistive loads (Fig. 23). An alternative method has been proposed by [Rodgers et al. 1995]: A known (measured) direct current through the signal coil is interrupted and the resulting transient response of the seismometer recorded. The generator constant is determined from the amplitude of the pulse; at the same time the free period and damping can be determined from its shape. Other methods applicable to this sensor type are mentioned in section 7.5.