First of all, I am not an expert on magnetism, so this is more of an additional question than answer (cannot add pictures to comments, so thats why its here).
In the case of ferrous materials they generate an magnetic field inside material (ok?).
Opposite signs attract each other (right?).
the position of the spring happens to be the local minimum of potential energy by symmetry principle (or you can actually calculate this).
all the other phenomena are just corrections to above phenomena (?).
If all above are summed together, the spring is just oscillating around a local potential energy minimum, because of the magnetic field, not because of the spring properties. This is also why the coin oscillates the same way.
Anyway could you comment on this, I would like to know where I went wrong (if anywhere).
I don't know much about the topic, but here are some research points you can get started with.
For strong magnetic fields, the most notable effect seems to be visual effects (source), called phosphenes (magnetophosphenes in the specific case of magnetic causes) caused by inductance of electric currents in the retina (source).
"Studies" seem to have suggested that 50T fields cause tissue damage, for unspecified reasons (weak source). I could not locate these studies. However, the implication is that immediate death / severe damage is not caused at even 50T fields (for reference, MRIs generally run in the 1.5-3T range).
There are related questions here:
There is an interesting discussion on Reddit:
There is also a field of study called bioelectromagnetics dedicated to biological effects of magnetic fields, which can serve as a good starting point for research:
"Transcranial magnetic stimulation", referenced in both the Reddit and Wikipedia pages, uses small fields in the range 1-10mT to affect the polarization of neurons in the brain.
It seems that the pattern of change of a magnetic field has a more pronounced effect than the strength of the field. Static fields do significantly less (or no) damage, while at high frequencies a weak magnetic field could certainly do significant damage, e.g. a microwave oven.
Primary causes of damage from non-static fields mostly seem to be due to heat, or due to induced electrical current; for example, from the ReviseMRI link above:
A more serious consequence of electric currents flowing through the
body is ventricular fibrillation (though these levels are strictly
prevented in MRI). ... As a general guide, the
faster the imaging or spectroscopy sequence, the greater the rate of
change of the gradient fields used, and the resultant current density
induced in the tissue is higher.
It would doubtless take an extremely strong magnet, higher than anything we could produce, to pull the iron out of your body (conjecture, no source). Note also that there is only about 3-5 grams of iron (something like 2 cm3) in the human body (source, unreferenced source), mostly bound to hemoglobin.
Count Iblis pointed out, in question comments, that there is a nice discussion of magnetars and strong magnetic fields here, which provides nice overviews and plenty of interesting information (although a bit dated):
From there:
Fields in excess of 109 Gauss, however, would be instantly lethal.
Such fields strongly distort atoms, compressing atomic electron clouds
into cigar shapes, with the long axis aligned with the field, thus
rendering the chemistry of life impossible. A magnetar within 1000
kilometers would thus kill you via pure static magnetism -- if it
didn't already get you with X-rays, gamma rays, high energy particles,
extreme gravity, bursts and flares...
As for long term effects of more commonly encountered field strengths, there is generally little association between magnetic fields and cancer (source, source).
I hope this helps. Sorry I do not know a direct answer. It certainly depends on more than just the field strength, however.
Best Answer
It is true that gold is diamagnetic and tungsten is paramagnetic.
The "magnetic susceptibility" of the materials is needed to quantify the effect.
Here is a table of magnetic susceptibilities:
http://www-d0.fnal.gov/hardware/cal/lvps_info/engineering/elementmagn.pdf
The problem I see with this technique is, as you can see by looking at the values in the table, that there are substances having susceptibilities hundreds or even thousands of times the absolute value of susceptibilities of gold and tungsten, not to mention ferromagnetic materials. In other words, a small impurity in the gold could give the same result as the entire bar being tungsten. The bar could be 99.9% pure gold and give the same result as 100% tungsten depending upon the other 0.1%.
Also, keep in mind that when tungsten faking has actually occurred, the gold bar has been drilled out and only a percentage of the gold bar has been replaced by tungsten. If, say 30% of the gold is replaced by tungsten, you will not observe a net attractive force on the bar, considering the weighted average of the opposite-sign susceptibilities of gold and tungsten.
There are two other important issues with the technique. Firstly, oxygen is paramagnetic. As you can seen in the table, oxygen has a much greater molar magnetic susceptibility than tungsten. When the gold bar is placed above the balance, air containing paramagnetic oxygen is displaced. This effect must be considered. Second, the magnetic susceptibility of the plastic envelope must be considered.
Also, the gradient of the magnetic field (how the field's intensity varies with space) determines the force, rather than the strength of the magnetic field. Magnetic susceptibility experiments are often part of undergraduate physical chemistry lab course. Typically, the sample is suspended from a microgram balance, the sample being between the poles of a large electromagnetic.
Checking for ultrasound echos coming from the interior of the bar at a gold/tungsten interface and measuring the speed of sound in the bar are techniques in use to rule out gold plated tungsten bars.