Researches on electrical rheometry

Angelo Secchi
Smithsonian Contributions to Knowledge
Vol. III, Art. 2
Washington City, 1852
Plate III
Lithography, 335 x 255 mm
Library of Specola Vaticana, Albano Laziale (Rome)

This study on electrical rheometry is the first interenational scientific publication by Angelo Secchi. The substantial paper, more than 50 pages long, was accepted by theSmithsonian Contributions to Knowledge in september 1850, during the exile of Secchi in the U.S.A. In Washington, at the College of Georgetown, Secchi become friend with his confrere James Curley, director of the Astronomical Observatory of the College, who introduced him to the local scientific milieu. Secchi did not deal with astronomy at the time, physics was his main interest. Few years earlier, in 1847, Secchi published, in a Roman magazine, a note with the title” About some experiences of Electric Rheometry”, in which he described the experimental evidence of some analytical developements carried on by Giovanni Plana- founder of the Astronomical Observatory of Turin and great mathematician- starting from the theory of the electric currents by André-Marie Ampére.
Ampére’s fame is mainly due to his contribution to science in the derivation of the relations between electricity and magnetism. He developed a mathematical theory explaining the phenomenon observed by Hans Christian Ørsted in 1820, the deviation of the magnetic needle of a compass from the magnetic north pole if it is putted close to a current carrying wire. The conclusion was that the electric currents generate magnetic fields, and that two parallel current carrying wires are mutually attracted if the currents have the same orientation, while they mutually repel if the currents have opposite orientation. This effect, known as “pinch effect”, and the discovery of these dynamic properties were of paramount importance for the development of the electromagnetic technology and were the fundamentals for operation of the electrical engine.
Secchi encountered a dynamic and stimulating scientific milieu in the U.S., and he met, among the other personalities, Joseph Henry, secretary of the Smithsonian Institute; Henry,researcher of electromagnetism and its applications, perfected the telegraph and he realized one of the first electric engines. It is easy to suppose that Henry would appreciate and maybe even used the work made by Secchi on Plana calculations, which otherwise would be unknown outside of Italian boundaries. Mutually, also Secchi took advantage by the knowledge of Henry’s work. The latter, using a thermoelectric battery, noted that different parts of the solar disk had different temperatures, and in particular that solar spots are colder than the neighboring regions. Some year later Secchi dedicated himself to this topic, designing and building the thermoeliometer in 1863, an instrument that could estimate the energy radiated by the Sun through a differential temperature measurement.
Even from this first work by Secchi one could see his passion for physics and for the “useful” applications of science, his ability in finding connections between several fields of research and of finding new branches of research, his propension and ability in the design and manufacturing of instruments, that were really useful to his studies.
The panel here decipted, accompanying the publication, shows the instrument manufactured by Secchi to carry on the experimental trials of the result analitically obtained by Plana. On the left side is visible the current carrying globe (Figure IV), inside this the magnetic needle (Figure. V), the electric battery (FIG. I), the telescope (FIG. VI) which is used to observe the movements of the magnetic needle from distance and with better accuracy, the rheostate (FIG. III e III bis), il galvanometer (FIG. II).

Francesco Poppi - Università di Bologna

Bibliography
Secchi, A.: “Sopra alcune esperienze di Reometria Elettrica”, Raccolta scientifica di Fisica e Matematiche, Anno III, N.24, Roma, 1847, pp. 385-394; Plana, G.: “Sopra le formole matematiche atte a risolvere i problemi relativi all'azione emanata dalle correnti voltaiche circolari”, Raccolta scientifica di Fisica e Matematiche", Anno III, 1847; Henry, J.: “On a Reciprocating Motion Produced by Magnetic Attraction and Repulsion”, American Journal of Science and Arts, Vol. 20, 1831, pp. 340-343; Mayer, Alfred M.: Henry as a Discoverer. "A Memorial of Joseph Henry". Washington: Government Printing Office. 1880, pp. 475-508; Altamore A., Ferrucci M., Poppi F.: 2015, “La ‘scienza utile’ di Angelo Secchi”, Giornale di Astronomia, Vol. 41, n. 3, pp. 31-33.