Making Resistive Magnets

Harnessing Electrons

First off, a quick explanation of what a resistive magnet is. It bears no resemblance to the things sticking to your refrigerator. Those are a different class of magnet altogether, called permanent magnets, made of alloys like Alnico – a combination of aluminum, nickel and cobalt. Due to the nature of those materials, they have a permanent but weak magnetic field. Our most popular magnets are resistive magnets, a type of electromagnet, which create magnetic fields using electricity. Unlike permanent magnets, electromagnets can be switched on and off. They can be very simple – you can make a basic electromagnet at home strong enough to pick up paper clips. But you can (and we do) make them a lot stronger, largely by pumping in more and more electricity.

In a nutshell, this is how they work. Electrons traveling through a wire (i.e., electricity) generate a magnetic field around them as they move, as shown below.

If you take that wire and coil it, you concentrate a lot of field in the center.

If you coil a wire twice, three times, a hundred times into a shape called a solenoid, you get an ever stronger field in the center area.

That's essentially the idea behind our resistive magnets, except that instead of wires, we use flat copper plates, or discs, piled one atop the other. The fabrication and stacking of the plates are designed to maximize the amount of electricity they carry (the greater the current, the greater the magnetic field it generates) while minimizing the chance that electrical heat will melt the plates and ruin the magnet.

These discs are the building blocks of our magnets; it takes hundreds of discs to make one magnet. The tiniest flaw in any one of these could impair the magnet or result in a costly malfunction.

So quality control is an obsession in the MagLab's Resistive Magnet Shop, where the coils are made. Every plate is checked, double-checked, and checked again throughout the magnet building process. Some very patient, persnickety perfectionists work in that shop.

"It's tedious," magnet technician Nicole Walsh readily concedes, "but if you realize that every single disc matters, then it just goes by faster."

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