[chris (OP)]

Eric would like some help:

I am building an electromagnet to experiment with PEM therapy. I used iron bolt (core), normal copper wires in rubber insulation (60m in 500 coils), applied 20 V. My ohm meter reads 5 ohms. So I guess I am running 4 Amp current through the electromagnet. But I managed to get only 2 Gauss reading on my EMF Detector (Samsung phone with Ultimate EMF Detector). What did I do that was wrong? My calculations show that I should get 2000 Gauss. But I'm getting only 2 Gauss. Help.

[Bored chemist]

Is the Gauss meter calibrated to measure DC fields?

[RD]

Is the sensor in the phone capable of measuring 2000 Gauss ?.
 It's designed to measure Earth's magnetic field  which is only ~0.5 Gauss.
2 Gauss may be as high as the meter/sensor will go.
NB: by applying a strong magnetic field you could magnetize metal in the phone, which will make its compass inaccurate.

[evan_au]

. I used iron bolt (core)

Magnetic materials go into saturation at lower frequencies, and don't "conduct" magnetism as much as you would expect.

And DC is the ultimate low frequency!

There are special alloys that are optimized to have a high permeability; these tend to be "soft" alloys, while a bolt is probably optimized for its tensile strength, and would not have been optimized for permeability. You may need to find a better material for the core of the coil.

If you are applying it to an area of the body, two coils on either sides of the limb (arranged as a Helmoltz coil) will provide a more uniform field. This uses just the coil to generate the magnetic field without relying on the bolt to concentrate it.

It appears that there are no clear studies showing what frequencies and waveforms are best for treating various conditions.
Higher frequencies would reduce magnetic saturation. I can't say whether they would improve health and wellbeing.

See: https://en.wikipedia.org/wiki/Pulsed_electromagnetic_field_therapy

[Colin2B]

It is also worth remembering that many formulae for field strength is that measured inside the coil and value even a short distance from the poles, over an air gap, will be reduced.

[chris (OP)]

There are special alloys that are optimized to have a high permeability; these tend to be "soft" alloys, while a bolt is probably optimized for its tensile strength, and would not have been optimized for permeability. You may need to find a better material for the core of the coil.

Evan - can you please explain the basis of this hard vs soft magnetic susceptibility / permeability effect.

[evan_au]

can you please explain the basis of this hard vs soft magnetic susceptibility / permeability effect

Each magnetic material has a characteristic maximum magnetization it can sustain before entering magnetic saturation. This is typically around 0.2-2 Tesla for iron alloys. To keep the magnetic field below the saturation level, you need a very large area of iron core to carry the magnetic field (not a skinny bolt).
See: https://en.wikipedia.org/wiki/Saturation_(magnetic)

Magnetic materials have microscopic magnetic domains, where the magnetic field occurs in alternating directions, canceling each other out on larger scales.
- For a "Soft" magnetic material, these domain boundaries can move easily in response to an external magnetic field, allowing the material to carry or "conduct" the magnetic field. These are good materials to use as the core of an electromagnet, since it easily carries the magnetic field generated by the electromagnet (and the "skinny" hysteresis curve shows relatively low energy losses).
- For a "Hard" magnetic material, the domain boundaries are frozen into the material, and don't move much unless a very strong magnetic field is applied to bring the material into saturation- and then they don't move back to the original position. This is not a good material to use as the core of an electromagnet, as it will mostly be delivering its pre-existing magnetic field, not the magnetic field generated by the electromagnet. If you do manage to magnetize it, you will lose a lot of energy magnetizing and demagnetizing it (a "square" hysteresis curve represents high energy losses).
See: https://en.wikipedia.org/wiki/Magnet
https://en.wikipedia.org/wiki/Magnetic_hysteresis

If you are pulsing the electromagnet, you need to beware of eddy currents. To minimize this energy loss, AC transformer cores are made of thin insulated sheets of iron, which have lower eddy-current losses than a solid lump of metal like a bolt.
See: https://en.wikipedia.org/wiki/Eddy_current#/media/File:Laminated_core_eddy_currents_2.svg