I have designed and built a small solenoid and... it doesn't exert any force on the steel bolt I am using as a plunger. I could freely move the plunger back and forth without any resistance.

  • I am using 0.2mm copper wire with a plastic coating.
  • The coil cross-section is 15mm x 3mm (inside radius = 2.5mm, ouside radius = 5.5mm)
  • I calculated that I could get about 1125 turns on the solenoid with a resistance of 14.7 Ohms. For the actual solenoid I measured 11.7 Ohms.
  • The solenoid frame is made of plastic.
  • The plunger has a 3mm diameter and the gap between the coil and the plunger is 1mm, of which 90% should be plastic and 10% air.
  • The overall radius of the solenoid is 5.5mm.
  • I applied 12V and the total current draw was 1.02A.
  • The solenoid became hot after 20 or 30 seconds, as expected.

I put the values into this calculator: http://www.daycounter.com/Calculators/Magnets/Solenoid-Force-Calculator.phtml

  • I = 1.02
  • N = 1125
  • A = 95 mm2 (PI x 5.5mm x 5.5mm)
  • g = 1mm

and it give a force of 78.598 N.

Clearly I have done something fundamentally wrong, but what?

Could it be the steel bolt?

I've looked for an iron substitute but couldn't find anything except old nails with a large taper.

What would happen if I tried a 3mm diameter neodymium magnet?

Thanks, Andy

  • \$\begingroup\$ For me it seems like the steel bolt is not magnetic. Have you checked with your neodymium magnet, if the bolt is attracted by it? Try using something which attracts the magnet. As you can see at the bottom of this page, being made of steel does not mean being ferromagnetic, and various stainless steels have different relative permeability. \$\endgroup\$ Aug 30, 2014 at 13:12
  • \$\begingroup\$ It sounds like you've done everything very well. I agree, it is likely the bolt. Try the nail to see if it is moved at all. Ideally find something which is likely to be steel or iron with some brown rust on it to help confirm it is not stainless steel. Nails, screws, nail-files, cheap tweezers, metal cheese graters, screw drivers, files, hammers, wire cutters, pliers, washing machines, cars, etc. are all possible candidates for steel or iron. You just need something to confirm the solenoid is working a bit. If you hold it suspended like a pendulum, it should pull itself towards iron or steel. \$\endgroup\$
    – gbulmer
    Aug 30, 2014 at 13:39

2 Answers 2


No force at all sounds like the bolt might be non-ferromagnetic. Simply test it with a magnet and make sure it is strongly attracted. 400-series stainless is only slightly ferromagnetic and 300-series stainless steel is almost totally non-magnetic under normal conditions.

If you sense the force is there (Luke?) but it is very weak, then keep in mind that the calculator you used assumes the total gap around the magnetic path is 1mm. If that sounds impossible to you- consider the following images from here:

(g = 1mm)


Or this one from the same site:


As you can see, the magnetic material of the core and armature forms a magnetic path that is only interrupted by a single small air gap (not counting the minimal clearance gaps required for the armature to pivot in the first image or the plunger to slide in the second image). The total reluctance around the path adds like electrical resistance, but the reluctance of the ferromagnetic materials is much lower than air, so the air gaps dominate.

  • \$\begingroup\$ Thanks. I was using a 300-series stainless steel bolt. Switching to a "alloy steel" and the bolt is rapidly pulled into the solenoid. Now the problem is that it has no holding force. \$\endgroup\$
    – Andy
    Aug 30, 2014 at 21:37

Could it be the steel bolt?

That would be my guess. If the bolt is stainless steel then it may not be very magnetic. See if your Neodymium magnetic is attracted to the steel bolt.


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