As described in this report, the limits on the strength of stable magnetic fields aredue to the magnetic forces on the conducting elements that tend to tear them apart:Magnetic Radiation Shielding: An Idea Whose Time Has Returned?Geoffrey A. Landis"The limit to the mass required to produce a magnetic field is set bythe tensile strength of materials required to withstand the magneticself-force on the conductors [8]. For the min-imum structure, all thestructural elements are in tension, and from the virial theorem, themass required to withstand magnetic force can be estimated as [9]:M = (rho/S) (B^2 V)/(2 mu) (1)where rho is the density of the structural material, S is the tensilestrength, B the magnetic field, V the characteristic volume of thefield, and mu the permeability of vacuum."You see the strength/density ratio of the material goes by the squareof the magnetic field strength. The conducting wire commonly used forproducing the electromagnets is made of copper because of its highconductivity and current carrying capacity. The tensile strength of copperis 220 MPa at a density of 8.92 g/cm³.The highest measured strength of carbon nanotubes has been 160 GPa ata density of 1.3 g/cm³. This is an increase of the strength to density ratioover copper of about 5,000.Then conceivably with this stronger material we could get highermagnetic fields strengths by a factor of the square root of this, 70;so to a magnetic field strength of 70 x 30 T = 2100 T.However, I have seen some reports that the square of the magnetic fieldintensity goes only as the tensile strength itself of the conducting material.In that case B^2 would only be larger by 800, so B itself larger by a factorof 28, so to 28 x 30 T = 840 T. Still this would be a major increase in the stablemagnetic fields attainable. Anyone have a reference that says whether it'sthe strength to density ratio or just the tensile strength itself that determinesthe intensity of the field that can be maintained?The nanotubes are only available so far at centimeter lengths. Still it wouldbe interesting to find out on tests with small fields if their use would allowmagnetic field strengths in the thousand tesla range.For the nanotubes to be used for this purpose they would have tocarry large amounts of current to generate the electromagnets. It hasbeen shown experimentally that they can carry thousands of times thecurrent of copper:Reliability and current carrying capacity of carbon nanotubes.APPLIED PHYSICS LETTERS, VOLUME 79, NUMBER 8, 20 AUGUST 2001."From the experimental results described in this letter wecan conclude that multiwalled carbon nanotubes can carryhigh current densities up to 10^9-10^10 A/cm2 and remainstable for extended periods of time at higher temperature inair. Furthermore, they conduct current without any measurablechange in their resistance or morphology, indicating thatthe sp2 bonds that are dominant in carbon nanotubes providemuch higher stability against electromigration than smallmetallic structures."http://www.rpi.edu/~ajayan/locker/pdfs/reliability.pdf [Broken]We can estimate the strength of the magnetic field we can obtain froma given current flow and wire size from the formula B = 2(10^-7)I/r,for B the magnetic field in Tesla, I the current in amps, and r thedistance from the center of the wire in meters, as described here:Magnetic Field of Current.For a 100 micron thick wire composed of carbon nanotube material,using a 10^10 A/cm2 current capacity, we could get 10^6 A of currentthrough. Then 100 microns away from the center the magnetic fieldwould be 2,000 T.One million amps is *quite* a large current. There are gas turbine electricalgenerating stations that put out 100 megawatts, enough to power asmall town, that at a voltage of 120 volts would put out about a millionamps. Imagine a generating station with enough power to run a town withall that power going into a single wire the width of a human hair!However, I'm wondering if these ultra high fields could be something that canbe reached by amateurs, if not as sustained fields then at least in pulsedfashion. Perhaps not as much current would be needed if a different arrangementwas made to create the magnetic field, such as a solenoid for example.This page gives the formula for the magnetic field of a solenoid:Solenoid Magnetic Field Calculation.You see that for a solenoid using an air core, there is a 4Pi factor in frontinstead of 2 as for the long wire case. So it's larger by a factor of about 6and you would therefore need this smaller amount of current.You could get a higher field with the same current by using a metal core:Magnetic Properties of Ferromagnetic Materials.The problem with this is that we are attempting to get the highest fieldpossible while sustaining the stresses. Using other metals for the core, thenthey have less strength than the carbon nanotubes and will fall apart atlower magnetic field strengths. We could use the nanotubes also for the corebut they give little in the way of higher permeability.For creating a short pulse of high current, this amateurs page describes getting25,000 amp pulse from a silicone controlled rectifier (SCR):The PowerLabs Solid State Can Crusher.And this page claims 70,000 to 100,000 amps can be reached in a short pulse:Coin Crusher.http://webpages.charter.net/tesla/crushed_coin.htm [Broken]Experiments at very high magnetic fields are very important fortheoretical studies. It is likely the nanotubes could withstand thehigh stresses induced by the magnetic fields at even higher strengthsthan 2100 T for short times, especially for nanotubes chosen to be lowin defects to have the highest strength. Then carbon nanotubes may bethe ideal material to use for producing ultra high magnetic fields fortheoretical work.Bob Clark