electromagnet stuff

Since you mentioned 12 volts before, if you wind that coil with 20 gauge film insulated magnet wire( about 1000 feet; coil will be approximately 26 layers at 88 turns per layer), you'll have about 12 watts into the coil (will draw about 1 amp and if you care, the inductance will be about 84 milliHenry) and get about 400 gauss at the center of the coil with no iron in it. Iron will increase the field. This should be in a safe continuous duty temperature range. To push things you can drop the wire to 18 gauge which increases the center field to about 800 gauss, but will put 45 watts into the coil. That baby will likely get HOT! If this is too weak then you've got to go with winding the coil with tiny copper tubing use low voltage and heavy current and run water through it to take the heat away. Any way to remove heat will help such as fans or heat sinks. But the problem is the heat is going into the center of the coil where air currents or heat sinks can't carry it away. Why not use another permanent magnet as a "pusher"? They'll be plenty strong.

Good luck!

Benj

The problem is that the flux at the center of the solenoid is not of importance. The flux at the end of the solenoid is more important for a relatively short device. This appears to be of the order of 0.00013 T or 130 Gauss, at maximum (distance 0 from near end of solenoid) This is considerably less than the 400 gauss correctly found for a long solenoid.

). Adding iron will modify this all to hell as then one doesn't have the same partial leakages and an estimate would be roughly dependent on the external air gap, ignoring the negligable iron mmf partial leakage pathsso an estimate based on a 5 inch average air path length would be about 0.23 Gauss.This may be sufficient for the purpose but it is considerably less than the 400 Gauss based on along coil no iron case. It appears that, for this application, a "bar" must be used rather than some other shape which minimizes the external air gap. That is too bad as it certainly limits what can be done with the given size and thermal constraints.

Don Kelly snipped-for-privacy@shawcross.ca remove the X to answer