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I'm attempting to build a crystal radio,

enter image description here

and I have an old telphone cable with four copper wires, similar to the one pictured.

enter image description here

I would like to use it to build the coil for the set, and I was wondering if it would be possible to use all four wires rather than stripping the entire thing to get one.

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  • \$\begingroup\$ I suspect the insulation will severely limit the number of turns you can add? \$\endgroup\$ – copper.hat May 22 '15 at 4:03
  • \$\begingroup\$ Sadly, it will. With that limitation aside, I wonder if the other three wires (assuming I only use one) will have an adverse effect. I also wonder how it will behave if I use all four wires in parallel. \$\endgroup\$ – WHY May 22 '15 at 4:19
  • \$\begingroup\$ I suspect that wiring them in series (maintaining the correct coil direction) will be of greater value? \$\endgroup\$ – copper.hat May 22 '15 at 4:33
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Yes it will work. Many transformers are wound the same way. Each colour wire will be a winding on the inductor - connect them in series to create the largest inductor.

You will need to use a variable capacitor to tune the frequency with this wire, rather than the picture shown which uses variable inductance.

Also the wire will be physically difficult to wind, as mentioned.

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  • \$\begingroup\$ Thanks for also mentioning the need for a variable capacitor. That sets me in the right direction. \$\endgroup\$ – WHY May 22 '15 at 4:48
  • \$\begingroup\$ Quick question: I've seen videos where they seemingly adjust the frequency by adjusting their handmade razor diode [youtube.com/watch?v=4-X1RuMQZQ8]. Does that have the same effect as an adjustable capacitor? \$\endgroup\$ – WHY May 22 '15 at 4:52
  • \$\begingroup\$ Yeh that razor diode has a lot going on. The body of the operator will give a large number of antenna-like and capacitive effects. \$\endgroup\$ – teambob May 22 '15 at 5:00
  • \$\begingroup\$ I was just thinking the most critical thing to getting a crystal set working in my experience is that the impedance of the ear piece must be high. Either get a crystal earpiece or use an audio transformer. \$\endgroup\$ – teambob May 22 '15 at 5:01
  • \$\begingroup\$ Would connecting it to an amplifier work as well? \$\endgroup\$ – WHY May 22 '15 at 5:46
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This is the wrong type of wire.

You need to use enameled wire for an adjustable inductor of that type, because you need to lightly sand off the insulating coating in the area which the wiper will contact.

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