At the end of the Ninteenth Century, one exciting technical development was the study and application of "Hertzian waves." Physicists learned that the intimate link between electricity and magnetism allows invisible waves to be generated and, some distance away, detected.
Among the instruments in the Cabinet of Physics are quite a few associated with this revolution, which led to the "wireless" or "radio" industry. Today's video portrays the coherer, a key device in the detection of radio waves.
The coherer is a glass tube that holds iron filings or other metal. Ordinarily it is a poor conductor; when it is in circuit with the Grenet-cell battery on the right, a galvanometer detects negligible current flow through the circuit.
But if a suitably powerful Hertzian wave passes through the room, the tiny filings can be jerked into alignment. Suddenly the coherer is a good conductor, and the needle of the galvanometer is deflected.
Supplying the invisible radio wave are the supporting players in this drama, seen on the left: Leiden jars, the wheel-like Wimshurst machine that charges them, and the brass spheres of a spark gap.
The coherer is awkward to use; after each detection, one must tap the tube to jiggle the metal filings, so they again become a lousy conductor. Nevertheless, this invention enabled the first generation of wireless to move from laboratory apparatus to commercial success. I imagine that many of the Italian students witnessing this demonstration went on to careers in the new radio industry.
The Foundation for Science and Technics, or Fondazione Scienza e Tecnica, of Florence, Italy, has made available many videos exploring the Cabinet of Physics, a large collection of antique scientific demonstration instruments. The Foundation's homepage may be found here, and its Youtube channel, florencefst, here.