A question about radio recievers

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In principle, it's possible to hear AM radio with earphones connected directly to the antenna - if the permanent magnets in the earphones are replaced with non-magnetized ferrite cores. Then, the magnetic force is proportional to the square of the current so the membrane (and whatever else can vibrate) is vibrating with the audio frequency.
 
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  • #27
sophiecentaur
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In principle, it's possible to hear AM radio with earphones connected directly to the antenna - if the permanent magnets in the earphones are replaced with non-magnetized ferrite cores. Then, the magnetic force is proportional to the square of the current so the membrane (and whatever else can vibrate) is vibrating with the audio frequency.
= a non linear element in the circuit. Tooth fillings can also account for people hearing radio transmissions but that's not a lot of use as a design feature of a receiver.
 
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Still, the Marconi Magnetic Detector was the first really useful one . . .

https://en.wikipedia.org/wiki/Magnetic_detector

435px-D%C3%89TECTEUR_MAGN%C3%89TIQUE_%28radio%29.jpg


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As for the frequency selective element, it's not necessary when you are not far from one real powerful long wave transmitter! 30 years ago, there was such a transmitter here, it was strong enouh to use a loudspeaker instead of earphones, without any amplification.
 
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marcusl
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In principle, it's possible to hear AM radio with earphones connected directly to the antenna - if the permanent magnets in the earphones are replaced with non-magnetized ferrite cores.
Not exactly. As stated previously, one needs a frequency selective element to isolate the desired broadcast from all others. This is called a "tuner" in the article you quote. Furthermore, Marconi's detector utilizes permanent magnets, and relies not on some purported square law force for detection but rather on the non-linearities in iron's hysteresis curve.
 
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marcusl
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Read the first paragraph in your link:
"It was one of the first rectifying detectors, able to receive AM (sound) transmissions. "
 
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Read the first paragraph in your link:
"It was one of the first rectifying detectors, able to receive AM (sound) transmissions. "
Perhaps I phrased it inelegantly. Giving it another go ....

Neither coherer nor Marconi's magnetic detector can demodulate audio. This milestone wasn't achieved until 1902 when Reginald Fessenden devised the electrolytic detector.
 
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marcusl
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Got it! Thanks for the clarification.
 

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