Friday, April 11, 2014

Edouard Branly Coheres

When CQ published a history of radio titled "Men of Radio" Edouard Branley was one of the three names in Bold beside Hetz and Maxwell.  Today he doesn't get that kind of top billing. If you know nothing else abotu his work at least know about the Branly coherer, which he developed around around 1890.
Branly was a professor of physics at the Catholic institute in Paris.He had not been studying radio but nerves. He had learned they are made of tightly packed fibers and had begun experimenting the properties of fine particles looking for an electrical corollary to what he's observed. His models began with the work of Temistocle Calzecchi-Onesti, who demonstrated in 1885 that iron filings in an glass tube will still conduct an electrical current. (It's also possible he had seen similar work by Alfred Varley)

Calzecchi's experiements ran from 1884 through 1886 and tested a number of metallic particles. The operation of Branly's coherer is largely based upon the electrical resistance offered by loose metal filings. The resistance decreases when current is applied between the terminals of the coherer.  Branly may or may not have been aware but the mechanism relied on the iron oxide on the surface of the filings. Those oxide layers are broken down when a voltage is applied causing the coherer to "latch" into its low-resistance state until the voltage ceases and the coherer is physically "percussed."  We'd probably just call it "tapping." regardless Branley detected resistance changes on the order of 1:1000.

It was actually Sir Oliver Lodge that named it the Coherer. It was a marked improvement on Hertz's spark detector. Branleymay not have been aware that his coherer generated hertzian waves, but Lodge certainly was. Using  Branlys cohoerer. Lodge went wireless signals 100 yards and because of that, Branly won the Nobel prize 30 years later. Branly died in 1940.