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I'm trying to build a homemade electromagnet to show my son. I took speaker wire and wrapped it around a stick maybe about 40 times. Then I used a C cell battery to power it, however I saw no magnetic effect when I exposed it to some screws.

Questions:

  • Is a C cell battery not enough power for this?
  • Is speaker wire okay for this kind of application, or should I use magnet wire?
  • Should I get longer wire to have more coils?

Thanks for any suggestions.

mj

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  • \$\begingroup\$ Have you tried wrapping the wire around something made of iron, like a nail? When you say speaker wire, do you mean the wire we'd normally use to connect speakers to a 'HiFi'? That is very low resistance. Roughly how long was the wire? Have you got a multimeter? Could you measure the resistance of your coil, and post that? \$\endgroup\$
    – gbulmer
    Sep 16, 2016 at 3:13
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    \$\begingroup\$ Here are three things that won't work: (1) screws that are stainless steel - test them with a magnet to ensure they attract.(2) speaker wire with the far end open - no current flows.(3) speaker wire with the far end shorted - equal current flows both ways so that there's no net magnetic field. \$\endgroup\$
    – glen_geek
    Sep 16, 2016 at 3:33
  • \$\begingroup\$ The magnetic force depends on total length of wire, diameter, current and permeability of the medium. ( screws are low "mu" with high nickel content ) \$\endgroup\$ Sep 16, 2016 at 4:06
  • \$\begingroup\$ Sticks make very poor electromagnets. Use something made of iron or steel (not stainless) instead. Magnet would be better than speaker wire because there's more wire and less insulation in the same amount of space. \$\endgroup\$
    – brhans
    Sep 17, 2016 at 1:19
  • \$\begingroup\$ Post a photo of what you built, because as @glen_geek points out, there could be a construction mistake. \$\endgroup\$
    – Aaron
    Aug 4, 2021 at 17:26

3 Answers 3

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  • Wrapping the wire around a large metal bolt can increase the pulling force by 10s to 100s of times. Using wood you won't get this same effect.
  • My own practical experience says that picking a bolt about 4 inches long and 1/2" to 1" diameter can give good results.
  • Use a regular iron/steel bolt. Brass, plastic, and some stainless steel bolts re not magnetic.
  • The magnetic field is proportional to the number of turns and the current. So more turns and more current will give more field.
  • The resistance is proportional to the length of the wire. The current is inversely proportional to the wire resistance ( I = V / R). Longer wire will have more resistance and therefore less current for the same input voltage.
  • The number of turns is almost proportional to the length of wire. So longer wire gives more turns. But its not exactly proportional because the as the number of wire layers increases the diameter of the cylinder you are wrapping around increases.
  • If you had an ideal voltage source, then doubling the length of wire would double the number of turns, but the current would be half, and so you would get almost exactly the same field.
  • But a battery is not an ideal voltage source. A typical D-battery (such as Duracell MN1300) has an output resistance on the order of 136-milliohm (see https://www.duracell.com/wp-content/uploads/2016/03/MN1300_US_CT1.pdf). In that case you will actually get the maximum field output if you match the wire resistance to your battery. So use an amount of wire that has a resistance of about 136-milliohm.
  • If the battery voltage is 1.5V, the wire resistance is 136mOhm and the battery internal resistance is 136mOhm then the current will be 1.5V / (2* 136mOhm) = 5.5A. The MN1300 datasheet does show curves for currents up to 2A, so 5.5A doesn't seem totally impossible for short periods of time.
  • At 5.5A you would need 22AWG wire or better (https://www.powerstream.com/Wire_Size.htm). Choosing the smallest wire diameter will let you get the most turns, so I would recommend either 22AWG or 20AWG wire. It can be hard to measure 136mOhms with a standard meter, but you can calculate how much wire you need form an AWG table. For 22AWG wire 136mOhms is about 8.4 feet (8 feet 5 inches).
  • The major difference between regular wire and magnet wire is that magnet wire has very thin insulation. You will get the highest number of turns in the smallest space (and thus the most field) by using the smallest possible wire. Speaker wire has bulky insulation and is not a good choice for that. Basically the thinner the insulation the better.
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  • \$\begingroup\$ "Choosing the smallest wire diameter will let you get the most turns". That's true in terms of fitting the turns into a given volume if space is the limiting factor. It's not true with respect to getting the most turns for a given amount of resistance. 136mOhms = 8.4 ft of 22 AWG wire (25.3 mil diameter). 136mOhms = 13.4 ft of 20 AWG wire (32 mil diameter). \$\endgroup\$
    – user4574
    Jan 6 at 2:23
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One thing you could do is up your voltage, C batteries only give around 1.5, same as AA, so if you put multiple AAs in a series you can max your voltage as their resistance(0.12) is a bit lower than Cs

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    \$\begingroup\$ A more realistic approach would be to make more turns. This would increase the magnetic field without the need to step up the voltage. \$\endgroup\$ Dec 1, 2018 at 7:27
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    \$\begingroup\$ The problem is that the OP has only a few turns of very low resistance wire. If it's, say, 0.1 Ω then he's trying to get > 10 A from a small cell. S/he'd need to add batteries in parallel to get that much current. \$\endgroup\$
    – Transistor
    Dec 1, 2018 at 9:08
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Is a C cell battery not enough power for this?

It's a bit weedy. More cells would be better.

Is speaker wire okay for this kind of application, or should I use magnet wire?

Thinner wire is better so you can fit more turns on.

Should I get longer wire to have more coils?

Yes. The more turns, the stronger the magnetism. Use thinner wire and many more turns. And as pointed out in the comments, wrap the wire around an iron or steel rod, such as a big nail.

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