3.2 Normal laboratory pendulum and seismographic pendulum in comparison  [ cont'd (b) ] - 3/3

Such seismographs are masterpieces of mechanical engineering. Even a small touch with a feather sets the heavy apparatus visibly in motion.

A significant progress in sensitivity (and ease of use) was the touch-free registration via electromagnetism. In this way the dynamo-principle was exploited. To the seismographic pendulum's "boom" a coil is attached, which is placed into a horseshoe magnet being in turn fixed to the ground. As induced voltage is proportional to velocity, one does not measure displacement with a modern seismograph, but instantaneous velocity. If one knows the so-called 'generator constant' of the system 'Induction-coil - Magnet' (compare chapter 4.3) then one can calculate velocity and displacement of the pendulum.

The seismograph equation is again differentiated and one concludes for v:

The following illustration reveals an overview for the varying resonance curves for forced oscillations for the 'normal laboratory pendulum' (left). For the 'classic seismograph' with a directional register and for a modern seismograph with a speed register. If one standardises the amplitude to the agitator-amplitude, the graphs reveals the so-called 'transfer-function': Ü ( w ) : = A _o ( w ) / a _o

Fig.30: Resonance curves

If one standardises the amplitude to the exciter's-amplitude, the graph reveals the so-called 'transfer-function':