It has a brass counter weight on the opposite side of the boom. Threaded rod allows for convenient adjustment. To the magnet was bonded a mild steel pole piece to help shape the magnetic field within the coil. An air gap exists between the sides of the pole piece and tube bore. Depending on the type of recorder used it may be necessary to eliminate IC4 from the circuit and alter the resistor values as connected to pins 2 and 6 on IC2.
Electronic feedback is achieved by monitoring the voltage generated by the seismic coil.
The differentiator is used to produce a signal that is proportional to the rate of change of the input signal. So with a constant DC level the output of the differentiator is zero. As the rate of change increases, the output voltage also increases. The input signal is differentiated for up to 100 seconds, the RC time constant. In Figure 4A the pendulum was free to swing with no damping at all.