From Ken Jones
In the initial part of the article on why grains might stick together to obstruct a flow (2 August, p 38), there was a reference to the observations made by Edouard Branly in 1890 regarding the reduction in electrical resistance of metal filings exposed to an electrical spark. This does more than a little injustice by failing to mention the work of one of Britain’s greatest 19th-century experimental physicists.
Oliver Lodge had observed, in 1889, that when two metallic surfaces were separated by a minutely small gap, they frequently fused together when an electric spark occurred nearby. During 1891 and 1892 he employed this principle in several different forms to detect radio waves. He observed: “This arrangement, which I will call a ‘coherer’, is the most astonishingly sensitive detector of Hertzian waves.”
When Lodge learned of Branly’s findings in 1893, he recognised their significance immediately. It was another example of his coherer principle. Lodge proceeded to develop it as the first real radio detector. It was used widely (including by Marconi) well into the 20th century. It is perhaps a little unfortunate that it became known as the “Branly coherer”, suggesting that Branly invented both the device and the name.
It is not true that “no one ever got round to explaining the bizarre electrical behaviour of the filings”. Over 100 years ago Lodge provided an explanation for the phenomenon along the lines referred to in the article, namely that the spark induces a radio-frequency electric current in the filings, welding together the points of contact to provide a conducting path for electric currents.
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The present findings, at least in the case of metal filings and electric sparks, provide no great improvement on the explanation provided by Lodge.
Sudbrooke, Lincolnshire, UK
