Girlanda’s Electromagnetic Seismograph: Italian Innovation in Twentieth-Century Seismology

The electromagnetic seismograph designed by Antonio Girlanda in the early 1960s represents one of the most refined examples of Italian seismological instrumentation in the post-war period. Conceived as an evolution of the Galitzin model introduced in the early twentieth century, it stands out for its simplified structure, accessible construction, and optimised dynamic response for short-period components. The horizontal pendulum, selected as the main image of this exhibit panel, forms the core of the instrument. It is characterised by a suspension system made of thin metal strips, ensuring nearly frictionless rotation and allowing the mass to oscillate with remarkable sensitivity. The mass contains four induction coils, easily interchangeable and designed to vary the electromagnetic amplification without altering the system’s inertia. Copper vanes, placed within a permanent magnetic field, provide constant magnetic damping, limiting free oscillations and stabilising the pendulum’s response. The electromagnetic operating principle, derived from the Galitzin seismograph but rendered more essential, made it possible to achieve high dynamic amplification while keeping construction requirements moderate. This allowed Girlanda to devise an instrument that could be entirely built within an ordinary university mechanical workshop, while still ensuring performance fully comparable to industrially manufactured models. The simplicity and practicality of the design facilitated its adoption in several Italian seismic stations. The vertical pendulum, also developed by Girlanda and documented in one of the secondary images, completed the three-component system for recording seismic motion. The combined use of a suspended mass, spiral spring, and magnetic damping enabled the acquisition of the vertical component of ground motion with dynamic characteristics similar to those of the horizontal pendulum. Coupling the pendulums to moving-coil galvanometers equipped with reflective mirrors allowed optical recording on photosensitive paper, producing continuous, high-quality seismograms. Girlanda’s instruments operated for decades within the seismic network of the University of Messina, generating records that today possess great historical and scientific value. Their significance lies not only in their technical performance but also in the ingenuity with which advanced instruments were made accessible within a local academic setting. They represent a crucial chapter in the history of Italian seismology, where artisanal precision and scientific inquiry intersect.

___ Maria Teresa Caccamo, Domenica De Domenico, Salvatore Magazù