“Electro-Atmospheric Apparatus”

Bullettino Meteorologico dell'Osservatorio del Collegio Romano, vol. 3 (1862), p. 18
295 x 215 mm
Biblioteca Storica centrale della Meteorologia Italiana, CREA-AA, Roma

In ‘700, following the experiences with electrostatic machines and bottles of Leyden, by analogy with the sparks, the idea that lightnings were electrical phenomena was affirmed. Moreover, in the last decades of the century, observations have been made on atmospheric electricity present in good weather conditions (when the electric gradient has an average value of the order of 100 Volts/m). Pioneers in this field were, for example, Benjamin Franklin, Alessandro Volta and Horace Bénédict de Saussure. In the nineteenth century the measurements of atmospheric electricity became current practice in almost all meteorological observatories and many types of electrometers (including recorders), probes, antennas and collectors were devised to explore the electric potential at different heights.
It is no wonder that even Father Secchi, in addition to the instruments of the meteorological Observatory, next to his magnetic observatory, installed on the roof of the Church of St. Ignatius,a turret with an “electric allotting” dedicated to the study of atmospheric electricity, illustrated in the page of the Bulletin represented here. Essentially the turret was equipped with three instruments. A lightning rod served to study electrical phenomena during thunderstorms. A link with a telegraph line that reached Castel Gandolfo and later extended until Anzio was used to detect the so-called telluric currents. Lastly, a Palmieri-type electrometer allowed to measure atmospheric electricity in clear skies. This apparatus consisted of a movable conductor, fitted on top of a spherical electrode, whose height could be modified by means of a pulley. The conductor was connected to a Bonhenberger dry battery electrometer but Secchi also had a Palmieri two-wire electrometer to calibrate the scales. The “Telluric currents” were instead studied by connecting a galvanometer to the long Telegraph conductor. Secchi noticed the correlations between the variations of the Earth’s magnetic field and the atmospheric electric field. For example, following a strong solar storm (Carrington’s event) that in early September 1859 produced phenomena similar to Northern lights even to the latitude of Rome , Secchi observed considerable variations in current in the conductor telegraphic while the magnetic instruments went out of scale. Many of Secchi’s observations confirmed those of other scientists: in fact, throughout the nineteenth century, studies on atmospheric electricity allowed to gather an enormous amount of data and observations (even on lightnings and their characteristics). But towards the end of the century none of the many theories yet proposed were able to interpret in a satisfactory way the mechanisms and the phenomena of atmospheric electricity. The latter were explained by valid reasons only from the beginning of ‘ 900, thanks to new discoveries (for example ionization) and within the framework of the nascent modern physics.

Paolo Brenni, CNR-IGG e Fondazione Scienza e Tecnica