From the Cabinet of Physics: Reflection and Invisible Waves
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In the 19th century, James Clark Maxwell's theory explained the inter-relation of electricity and magnetism. It also predicted that electromagnetic waves should exist. In the 1880s Heinrich Hertz worked in the laboratory to produce these waves, and to measure their properties.

The Cabinet of Physics can demonstrate experiments similar to some of Hertz's. The transmitter is a spark gap driven by an induction coil; the receiver includes a Marconi-style coherer (remember the coherer?).

The coherer is part of a circuit containing a battery-powered bell. So when waves from the transmitter arrive at the receiver, the coherer transitions from being a bad conductor of electricity into being a good conductor, and we hear the bell ringing. It keeps ringing until a sharp whack returns the coherer to its original state.

In today's video, the curators first use this setup to show that interposing a sheet of copper reflects away the waves from the transmitter. The receiver's coherer detects nothing, and the bell remains silent.

Next the apparatus is arranged so the transmitter is no longer aimed at the receiver, but instead at the copper sheet. The receiver, also aimed at the copper sheet, rings its bell. By similar means, Hertz was able to demonstrate that conductive objects could reflect the invisible waves, just as a mirror reflects light.

In the 20th century, such waves became known as "radio" or "wireless." The principles demonstrated here lie at the heart of antenna design, using conducting materials to reflect and shape radio waves in a great variety of ways. Among other devices, the huge antennas of today's radio astronomy descend from Hertz's experiments.

The notion that a receiver might catch a signal bounced off a passive conductor also hints at the idea of radar–though to make radar work, technology well beyond the apparatus seen here would eventually be required.

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.


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