The last episode of BBC Four's series 'Victorian Sensations' aired on 5th June and included presenter Philippa Perry demonstrating one of Oliver Lodge’s pioneering experiments with electromagnetic waves using Branly coherers. The segment was filmed in the Science School museum earlier this year.
Winchester College's first physics teacher, William Bleaden Croft, corresponded with several leading scientists of the day, including Lodge, and was directly involved with these developments. He was, we think, the first person in England to perform the demonstration at the Physical Society of London in 1893. We still have his original instructions and equipment and it was these that brought 'Victorian Sensations' to Winchester.
After a certain amount of trial-and-error the Physics Department (with assistance from former Head of Physics, Dr. Martin Gregory) were delighted to be able to reproduce the experiment for the film crew. The key piece of kit is a glass tube filled with metal filings, known as a 'Branly coherer' after its French inventor. It was an early device for detecting radio waves, which Hertz had first intentionally produced in 1887.
The tube is connected to a small battery, but very little current flows since the filings are separate from each other. Next, a spark is produced (in this case by an induction coil), generating electromagnetic waves. As these pass through the tube their electrical field causes the metal filings to clump together or “cohere”, which greatly reduces their resistance. The increased current is visible on the galvanometer and will persist until the filings are “decohered” by tapping them. With a bit of ingenuity this can be used to send wireless messages in Morse code; Marconi used similar coherers in his early detectors.
The experiment can be seen on BBC iPlayer at 04:24 - 05:50.
Croft’s coherers and notebooks are one of the highlights of our unique collection of historical scientific apparatus, some of which will be on show at the Winchester Heritage Open Day event in September 2019. Tours are free but booking is required and tickets will be available via the Heritage Open Day website from August.
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