I've been trying to build a coilgun, so I winded a cable around a 7cm long pvc tube (the diameter is 2 cm). The cable is composed of about 30-35 parallel wires so I calculated the general resistance of the cable (using ohm's second law for each wire and than the formula for the resistance of parallel resistors) and it turned out to be around 0.017 ohms.

After that I measured the currents and the voltages, this resistance value didn't make any sense (for example I had 1.2 V and only 0.2 A) so I tried to use resistors with known resistance to find out the internal resistance of the multimeter which I was using. This turned out to be, on average, 5.96 ohms.

I then measured again currents (considering the sum of multimeter's and cable's resistance) and voltages and this time they were normal, but only for low voltages (less than 9V). When I pushed the circuit over this threshold, the current were abnormally low (For example, when I used 23V I only had 1.9A).

The coil also generates a very weak camp. I used an app for smartphones (I know that isn't a good instrument but I used it only for reference) and it measured about 40 microteslas with 1.5A, instead a little bar magnet generated about 50 microteslas (it is also visible, because the coil can only move magnetic needles, instead the bar can pull some paperclips).

The coil has 72 turns, I don't know what went wrong. I hope you can help me. Thanks in advance.

The cable which makes the coil

  • $\begingroup$ What are you using to supply the 23 V? Is there a current limiter? $\endgroup$ Nov 15 '21 at 15:26
  • $\begingroup$ Are your power supplies AC or a well filtered DC? You may be getting reactance from the coil. In measuring resistance you may be seeing a voltage drop on the ammeter, or current going through the voltmeter, depending on how the meters are connected. $\endgroup$
    – R.W. Bird
    Nov 15 '21 at 15:36
  • $\begingroup$ A coil tends to attract a ferromagnetic material at either end. Under what conditions would you expect this to act like a gun? $\endgroup$
    – R.W. Bird
    Nov 15 '21 at 15:46
  • $\begingroup$ The power is supplied using different combinations of 1.5V and 9V batteries (Some are partially low, so I measure the voltage before every test). The wires are tightly packed inside an insulating coating, I'll try to edit the post and add photos of the coil. $\endgroup$
    – Phys
    Nov 15 '21 at 17:12
  • $\begingroup$ (a) Thanks for the picture. It's not a cable you're using but a multi-strand wire! (b) Using the figures you've given, the magnetic field strength at the end of the coil on its axis should be in the order of 0.5 mT per ampère of current. (c) The current through your wire is limited by the internal resistance of the power supply. Having a very low resistance coil won't help, and may well damage the power supply, or flatten the batteries quickly – without giving you the hundreds of amperes that you need. $\endgroup$ Nov 15 '21 at 19:33

Your power supply is not strong enough to pump enough current though the coil to make a big field. Furthermore, when you jack the supply to the max, a self-protection circuit inside it called a foldback kicks in and chokes off the current flow to a low value to protect the power supply from a too-low load impedance.

You need a very beefy power supply capable of several hundred amperes surge current to drive a railgun magnet.


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