Upper boundaries of the strong earthquake motions recorded until now indicate almost constant value of 20-30 kines (cm/see) over the period range from about 1 second to 20 seconds. Accordingly, it may be thought that a velocity seismometer is suitable to observe the strong earthquake motion. Weak ground motions which occur frequently are also valuable to research the properties of strong earthquake motions. Therefore, the author made a velocity type seismograph which can record the earthquake motions of 100 kines to 0.01 kines with high fidelity in the period range from 0.05 seconds to 40 seconds.
The main part of this seismograph has a coupled pendulum which was designed by T. MATUZAWA in 1938 and was improved by the author. The whole system of the coupled pendulum is immersed in high viscous silicon oil. The relative displacement of the pendulum is transformed to electric voltage by means of magnetic flux to which a temperature compensating circuit with a thermistor is connected.
