Q&A: Is there any way to make your own experimental telephone out of materials including carbon bits and a speaker?

February 8, 2013 | By

Question by Mr. MacArthur: Is there any way to make your own experimental telephone out of materials including carbon bits and a speaker?
I saw on a television program years ago in which a man yelled into his home-made reciever, and his voice was then reproduced by a large speaker connected to the reciever. I'd like to know how he did it. Thanks!

Best answer:

Answer by Kes
The original telephone mouthpieces used a small funnel to concentrate the voice onto a thin metal diaphragm that had a metal part (pointer?) that could compress carbon bits more or less as the diaphragm vibrated. As the carbon bits compressed and decompressed their resistance to the flow of electricity varied in proportion to the compression causing current to flow in an external wire that replicated speech as an analog. The external wires were connected to the ear piece of the person receiving your message. The ear piece contained an iron rod wound with many turns of very fine insulated wire forming an electromagnet. When another very thin round diaphragm of sheet metal (attracted to magnets) was held very near the electromagnetic pole but not touching it, it vibrated in proportion to the current changes produced by the mouthpiece; producing sound waves. A power supply (DC battery) was included in the circuit to cause current to flow.

I would first try by placing the mouthpiece diaphragm horizontally over a dish of carbon bits with a pointer connected at the center of the diaphragm dipping into the carbon bits. Gravity will hold the bits in contact with the pointer. The carbon bits may rest on a conducting plate which is lead out of the dish using a connected insulated small diameter wire. A second wire would be connected to the pointer or diaphragm if they both conduct electric flow. Connect the two wires of the mouthpiece to the two wires of the ear piece including your battery in series in the circuit. Use very small voltage by adding 1.5 volt cells in series as needed so as not to burn out the ear piece. Experiment with the pointer and confinement of the carbon bits to optimize the rig.

Incidentally, both the mouthpiece and earpiece can be made of wire coils rather than use carbon bits. Try wiring two earphones together (without a battery) and you can talk in one and hear at the other. Good luck.

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