Weighing magnetic properties

It's a New Year and there are lots of things to do at work before the students get back in any numbers. There are still summer students and research students here, and in the last couple of days I've been working with a summer student on getting a new piece of equipment running for our Experimental Physics paper – the Gouy Balance for measuring magnetic susceptibility. 

Magnetic susceptibility is a measure of how magnetically responsive a material is – how much it magnetizes when placed in a magnetic field. Materials can be categorized as diamagnetic, paramagnetic or ferromagnetic. Paramagnetism describes a material that magnetizes with the applied magnetic field – that is, it will be attracted to a region of high magnetic field. A ferromagnetic material goes beyond this – not only is it attracted to a region of high magnetic field it retains its magnetization even after being removed from the field. Iron is the obvious example – once you magnetize it it will stay magnetized. Diamagnetism is the opposite of paramagnetism – a diamagnetic material will magnetize in a direction against the applied magnetic field and therefore be repelled from a region of high magnetic field. Water is a easy-to-get-hold-of example. 

You can demonstrate the diamagnetism of water with some rather simple apparatus. Get a short stick, skewer a couple of nice, ripe tomatoes at each end, and hang the stick by a thread from its centre. Adjust the tomatoes so that the stick is roughly horizontal when you hang it. Wait for it to settle down then take a strong magnet (a rare-earth magnet is best) and move it close to a tomato. The tomato will be repelled.

The Gouy Balance measures susceptibility in a broadly similar way – by measuring the force on a sample of material when it approaches a magnet. With the equipment we have, we actually doing the opposite – we have a magnet on a sensitive balance, and we look at the change in the weight of the magnet as a sample of material is brought towards it. With a paramagnetic material, as we lower the material toward the magnet, the magnet is attracted (slightly) to the material, and the weight recorded on the balance is reduced. The size of the reduction lets us calculate the susceptibility. 

The changes aren't big – with our test sample of titanium powder this morning our magnet's measured mass changed from 184.142 g to 184.014 g as the material approached – a change of 0.128 g, or about 0.07%. One certainly wouldn't feel the difference if one were holding the magnet, but the balance is sensitive enough to pick up the change. 

It's a neat little apparatus and will be fun to play with. And it comes with a demonstration of magnetic levitation with pyrolytic graphite


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