Magnitude of Magnets: From the Human Brain to Outer Space

The smallest magnetic fields and field variations can be detected using a device called a SQUID (Superconducting Quantum Interference Device). These devices, which are probably named for their squid-like appearance, can detect fields as small as 20 pG (1 picoGauss = 10^-12 Gauss)

Moving up the scale, we encounter the magnetic fields produced by the human brain. These fields, which range from 1 to 10 nG (1 nanoGauss = 10^-9> Gauss), are created by the electrical activity of our nerve cells. This is opening up new and exciting areas in the field of medical diagnostics, like MEG (magnetoencephalography) and MCG (magnetocardiography), that can help to detect diseases earlier, and are completely non-invasive.

Next up, in the realm of astronomy, NASA's Voyager 1 spacecraft (the farthest-ranging man-made object, which passed through to interstellar space in 2012) measured magnetic fields of 1 to 100 µG (1 microGauss = 10^-6 Gauss) in the heliosphere, with magnetic “potholes” measuring around 100 nG, caused by turbulence as the solar wind meets the interstellar medium.

Coming back to Earth, let's look at the magnetic fields we encounter in everyday life. As we know, magnetic fields follow electric fields like a shadow. A normal toaster produces a magnetic field of 600 µG to 6 mG (1 milliGauss = 10^-3 Gauss), and high-voltage transmission lines around 20 mG at 30 m. A foot away from a microwave oven you can expect to see a field of 40 to 80 mG. For those of you who have seen a magnetic tape (we know exactly how old you are!), it has a field strength of 240 mG at the reading tape head. This is comparable to the magnetic field of the earth itself, more on which below.

The Earth has its own magnetic field, which varies from about 250 mG at the equator to about 600 mG at the poles. It might look tiny, given that it's a big place! In fact, at first glance, it doesn't seem to show any magnetic properties at all (we don't see the steel in our buildings and ships shoot over the horizon towards the north pole), but if we hang a magnet so that it's able to rotate freely, it always aligns itself in a generally north-south direction. This feature was very useful to early navigators during their exploration of unknown lands and oceans. The field is vitally important to life on earth in general as well – it is because of this field that we are shielded from much of the harmful radiation from the sun and cosmic rays that bombard our planet all the time. In fact, the enormous energies captured and deflected by our magnetic field can be seen in the higher latitudes as spectacular displays of aurorae, also called Northern Lights – a nightly show of shimmers and curtains of light that occur when charged particles trapped by the magnetic field enter the upper atmosphere and crash into air molecules. Aurorae are especially bright during periods of sunspot activity, which are areas of reduced temperature caused by high magnetic fluxes of around 1,500 G on the surface of the sun.

Magnets can also impact electronics. People with pacemakers, for instance, are advised not to be in the vicinity of magnetic fields greater than 5 G. This is generally avoidable, but what happens if someone with a pacemaker tries to get ice-cream from the fridge? The familiar fridge magnets have a field strength of around 50 G, surely too much exposure for a pacemaker? This brings us to an interesting concept called the Halbach array, named after physicist Klaus Halbach, which uses a special arrangement of magnets to effectively double the flux on one side while neutralizing it on the other. This allows the magnets to attach to the fridge door while showing almost no magnetism on the other side (this can be easily tried out with a paper clip – it gets attracted only to the fridge side of the magnet but not the other).

Most magnets that we use in everyday life – in music system speakers, for example – are in the range of 1,000 to 15,000 G (0.1 to 1.5 Tesla). They are commonly made from 3 kinds of materials, Ferrite (a mixture of Ferric oxide, Fe2O3, and another metal, e.g. Zinc, Strontium or Barium) being the cheapest and most common. Rare-earth magnets, made using Neodymium-Iron-Boron (also known as Neo magnets) or Samarium-Cobalt, are more powerful, topping out at around 1.5 T for Neo magnets. I have the pleasure of working with these beauties, but one must be very careful when handling them – a 5 cm magnet can put out a crush force of more than 100 kgs! You can visit www.suprememagnets.com to check out the wide range of different types of magnets that are available.

Even stronger magnetic fields can be made with electromagnets. These are created by having electricity flow through a wire coiled around a metal rod, typically iron, which enhances and focuses the magnetic effect. Typically, the higher the current, the stronger the magnetic field produced. However, there are practical limits, as electricity also generates heat, and with high currents the overheads associated with cooling requirements become prohibitive. One way to avoid heating is to use superconducting magnets, and these are regularly used inside MRI machines to produce fields of 5 to 7 T. At a strength of 16 T, an electromagnet can be used to actually levitate a frog!

The strongest continuous magnetic field that can be currently produced clocks in at 45 T, while another technology, the pulsed magnetic field, can reach up to 100 T – if one wants to continue producing it without destroying the equipment, that is! The strongest field ever produced by humans (which took down the equipment and part of the lab with it) was 1,200 T for a fraction of a second, and this was a controlled experiment! Researchers in US and Russia have produced even stronger fields, topping out at 2,800 T, using TNT to implode magnetic coils, but for obvious reasons, these are impossible to calibrate and study.

Of course, anything we humans can do, the universe can do better, and a discussion on magnets wouldn't be complete without mentioning the largest magnetic fields ever detected in the universe. Neutron stars, which are the remnants of supernova explosions, are thought to have a superconducting core and rotate at extrmely high spoeeds, generating magnetic fields of around 1 to 100 MT (1 MegaTesla = 10⁶ Tesla). Their cousins, magnetars, have even more crazily enormous fields of 1 to 100 GT (1 GigaTesla = 10⁹ Tesla). It’s good for us that they are very far away – one such monster halfway to the moon would wipe off the magnetic stripes of all credit cards on earth, and they would be lethal to life even at a distance of 1000 km!