An “Optical” Bench for the study of Electromagnetic Waves, by Francesca Damiani, University of Bologna

The discovery of electromagnetic waves, made in 1886 by Friederich Hertz, led the German physicist to deduce that the nature of these waves was the same as that of light waves. More detailed experiments were performed by Augusto Righi, founder of the Institute of Physics of Bologna. Righi’s purpose was to replicate the optics experiments to verify the properties of electrical oscillations, such as reflection, refraction, diffraction and polarization.

Figure 1: The bench for electromagnetic waves kept in the Museum of the University of Bologna

To do this, he developed the three-sparks oscillator, in order to generate oscillations of controlled wavelength.

Figure 2: A sulphur prism that can be mounted on the bench

Based on the same scheme of an optical bench, Righi developed the bench for electromagnetic waves: the three-spark oscillator was used as a generator, and a piece of silvered glass with a thin groove as a detector. In the central disk, it was possible to place different obstacles, such as prisms and lenses of sulphur, metal mirrors, gypsum and selenite crystals, on which the waves engraved.

Each piece of the optical bench can be replaced, in order to study waves of different wavelengths. The detector is mounted on a wooden arm that can rotate around the central disk, on which, thanks to a graduated scale, you can read the angle of rotation, to identify the position of maxima and minima, observed by the appearance and disappearance of sparks in the engraving on the glass.

Details about the instrument and the experiments are described in Righi’s 1897 book The Optics of Electric Oscillations.

Figure 3: The Holtz machine, an electric current generator used to charge the oscillators

In my research, that will be submitted in the Bulletin, I studied Righi’s handouts and books, and examined the instrument with its accessories that are preserved at the Museum of the History of Physics at the University of Bologna, and other original detectors and oscillators kept at the Museum of the History of Physics in Padua.

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