Better detection than the coherer


You can impose waves, and waveforms, onto current in lines. I’d never thought about the frequency of an electrical signal changing, but that’s what makes voices audible in telephones. Pretty obvious.

‘Detection’ of radio signals means changing their frequency down to a range to which a pair of headphones (and your ears) can respond. You can do it with a transformer, such as the one I’ve found here, in one of those Ladybird ‘achievements’ books for children. Listening for a signal transformed to audio is a more sensitive way of detecting signals than trying to line up metal filings in a coherer, such that their collective resistance drops, and getting a bell to ring. 

Marconi is credited for devising this first, and doing it, as near as I can tell, in 1902. It became standard shipboard equipment for a decade or so, gradually being overtaken by systems that did the trick instead with galena crystals or with the vacuum tube diode, which Ambrose Fleming rolled out in 1903.

The received wisdom goes that nobody, not even Marconi, was able to say why this funny transformer setup worked. It begs the question, of why he devised it.

The system consists of a primary coil, through which RF signals run, a secondary coil into which current gets induced, some strong magnets perfusing their flux lines through the whole system, and a slowly moving (by hand or by clockwork) core of iron wire.

The reason it worked was, more or less, that the iron atoms in the moving core would line up in conformity with the magnetic field, in the absence of incoming current into the primary. (Why the core had to be moving for this, I don’t know.) When a pulse arrived, the atoms would change orientation, behaving in the manner of the core of an electromagnet. That change was detected by the secondary coil. We’re taught to say that ‘the magnetic field collapsed’ at that moment, though it’s not clear to me what this means. As soon as this collapse happened — and here my reading varies — the listener heard a tone or a click. I don’t know which. Either way, Morse code was audibly detected. 

This system was known to contemporaries as the ‘magnetic detector’, or the ‘Maggie’.  You can see a picture of one of these at John Jenkins’ SparkMuseum, here.


Photo credit:  F.G. Goodall, Robert Ayton, illustr., The Story of Radio (Wills & Hepworth, 1968), p. 31. 

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