2013: Advent Chemistry: Magnetic balances

by on December 13, 2013
Secretly, this lady can fly.

Secretly, this lady can fly.

One of the more curious devices in a chemistry lab is the magnetic balance. This is an extremely precise balance – as in, accurate to the nearest hundred thousandth of a gram or closer – with an electromagnet underneath. The balance lives in a nearly airtight glass box, because the breeze of someone walking past will turn your data to gibberish. The vibration of leaning forward on your chair to look closer will upset it, for that matter.

You put a very small amount of a substance on the balance, shut the box, wait for it to settle down, and write down the weight. Then you switch on the electromagnet.

Compounds come in three flavours of magnetism: ferromagnetic, diamagnetic, and paramagnetic. Ferromagnets are permanently magnetised, like a compass, or anything you’d usually call a magnet. They’re called ferromagnets because they are magnetic-like-iron. Diamagnets and paramagnets are different. They only have a magnetic field in response to an externally applied field. Pretty much everything responds to a magnetic field in one of these two ways: it is either slightly attracted to the magnet, or it is slightly repelled by it. It either aligns with the external force, or opposes it.

What this means is that when you turn on the electromagnet sitting inside your extremely precise balance, the substance on top changes its apparent weight. It gets either “heavier”, or “lighter”. An ordinary balance can’t pick up these tiny changes, and they’re pretty much irrelevant to daily life, but if you have a big enough electromagnet you can levitate a frog. Because frogs are diamagnetic.

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