When a pendulum type vibrograph is used in measuring vibration, the longest measurable period is determined by the natural period of the pendulum. Therefore when the measured vibration is supposed to have a long period component, the natural period of the pendulum must be made long accordingly. It is not possible, however, to make the natural period of the pendulum itself unlimitedly long because the size becomes prohibitive and the measuring accuracy degenerates.
The author proposes the “servo-vibrograph” in which the pendulum itself is comperatively small and with a short natural period but the apparent natural period of the vibrograph is made long by the servomechanism incorporated with the pendulum. This paper presents considerations on various conceivable constructions of the servo-vibrograph which can achieve the above objective, i. e., the elongation of the natural period. The torsional servo-vibrograph designed and built by the author is described in relation to the above considerations, and with the experimental results which are compared with the theoretical calculations pointing out some of the salient natures of its design. The built servo-vibrograph employs a pendulum whose natural period is 1 second. Experimental data were taken when the natural period was lengthened 2, 3 and 7 times longer by the servomechanism. Comparison is made between this servo-vibrograph and other conventional types of vibrographs.