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So, as far as I understand, speakers are basically coils coupled with permanent magnets, then when a current is run through the coil it creates a magnetic field, pushing out against the permanent magnet, moving the diaphragm that's attached to the permanent magnet and creating sound waves.

What I'm interested in is using a coil to do something similar but instead create human perceivable pressure. For example, something to lightly but noticeably push against your finger at a constant rate (not something that constantly moves back and forth like a speaker diaphragm).

If I just make a small coil out of some magnet wire and hook it up to a battery, I think that the magnetic field created by the wire would create a constant pressure against the magnet with the current being proportional to displacement. Is this correct?

If that's right then my real question is if this is possible to do without using a permanent magnet? I know you can create an attractive force to metal with a coil but is there a way to create a repulsive force?

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  • \$\begingroup\$ "... create a constant pressure against the magnet with the current being proportional to displacement." Force will be proportional to current. Pressure is force / area. \$\endgroup\$
    – Transistor
    Commented Jul 29, 2016 at 19:02

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What you are describing is commonly known as a "solenoid". It's a readily available electrical components something like a relay. Go to www.mcmaster.com and search on "solenoid" and you will see lots of commercially available varieties, though most are probably too large for your specific use.

The solenoid is a coil of wire (actually many turns) formed around a circular tube. An iron, but un-magnetized, rod slides into the tube. When the coil is energized the rod (often called the "plunger") will move in or out of the coil depending on the specific design.

So, yes, the mechanism you describe is indeed possible. However, one problem you may run into is battery life. Solenoids are typically current "hogs". In order to maintain the magnetic force on the plunger, the current must remain flowing thru the coil. While current is flowing battery power is being consumed.

If battery size and life is an issue you may have to apply some cleverness to your design and come up with some kind of latching solenoid, probably with two coils - one to push the plunger into the finger and another to pull it back. This way you will only need to energize the coil, and consume battery power, when the plunger is moved, rather than the entire time it is engaged against the finger.

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  • \$\begingroup\$ Does the rod necessarily have to be inside the tube? Would it work just as well with a flat bit of metal resting above the solenoid? \$\endgroup\$
    – Indigo
    Commented Jul 30, 2016 at 21:27
  • \$\begingroup\$ Additionally, is there a reason for all solenoids being a long tube shape? Are there ones that are flatter that I just haven't run across? \$\endgroup\$
    – Indigo
    Commented Jul 30, 2016 at 21:34
  • \$\begingroup\$ If you put the "flat bit of metal" above the solenoid as you describe, you would be making a standard relay without the contacts. The moving part of the relay is called the "armature". This arrangement is sometimes used as a mechanical "actuator" instead of the traditional tubular solenoid. The shortcoming of the armature design is that the actuating force is relatively weak compared to the cylindrical solenoid form. \$\endgroup\$
    – FiddyOhm
    Commented Aug 1, 2016 at 22:20
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    \$\begingroup\$ The reason for the long tube shape is it's the easiest to manufacture and it is efficient magnetically. There are commercially sold solenoids with square plungers. I have seen specialized solenoids with other shape plungers. It's not out of the question that you can make your own. If you decide to roll your own, get a copy of this classic: "Solenoids, Electromagnets and Electromagnetic Windings" by Charles Underhill. Easily available on the Internet. It was written in 1914, and solenoids haven't changed much since then. \$\endgroup\$
    – FiddyOhm
    Commented Aug 1, 2016 at 22:59
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You can either use two coils of wire, or one coil of wire and a permanent magnet. Both will work.

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  • \$\begingroup\$ The repulsive force requires that you ORIENT the permanent magnet and the coil, not just set them side-by-side, of course. \$\endgroup\$
    – Whit3rd
    Commented Jul 30, 2016 at 5:20
  • \$\begingroup\$ Very nice :) +2 for two coils, -1 because the OP specified "without a permanent magnet." \$\endgroup\$
    – EM Fields
    Commented Jul 30, 2016 at 5:22
  • \$\begingroup\$ @Whit3rd: Did you, somehow, miss the subject line and the OP's following text? What permanent magnet are you talking about? \$\endgroup\$
    – EM Fields
    Commented Jul 30, 2016 at 5:28
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One way to create a repulsive force is to use a diamagnetic material for the coil's magnetic field to push against, another is to use @user96037 's excellent double coil suggestion.

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