To begin with, a curiosity::

foto della dinamo di Pacinotti

A thin (ferromagnetic) iron pipe is placed so that it can run up and down a guide. It spontaneously goes to the area of maximum intensity of the magnetic field which for a ring magnet is evidently not on the plane of the ringís surface, but slightly above it.

ritratto di Pacinotti con la sua prima 'Macchinetta'

Now letís study the forces between magnets and different materials.
A photograph of magnetic action through non-magnetic materials.

To see click on the photo

schema di un indotto ad 'anello' con le spazzole commutatrici

Now letís see two experiments in which we measure the force that a magnet exerts on two materials having quite different magnetic properties. We use an analytical scale to the milligram. In the foreground a small graphite cylinder, a pure aluminium rod and a powerful samarium-cobalt magnet. On the right plate we have placed a light plastic glass upside-down, the purpose of which is to keep the graphite sample away from the plate which, although made of brass, is galvanized so it can be attracted by a magnetic force.
To see the experiment click on the photo

A force of repulsion of some ten milligrams is evident. The graphite is diamagnetic!
An application of this property is to be seen on the page devoted to levitation phenomena: click here.
In diamagnetic materials a magnetic pole gives rise to a weak pole of opposite sign of induction proportional to the intensity of the inducting field.


foto di un altoparlante

And now an experiment in which we show what force a magnet exerts on pure aluminium.

To see the experiment with the aluminium rod click on the photo.

The aluminium rod is attracted weakly. Pure aluminium is paramagnetic.

We have not repeated the experiment with a ferromagnetic material since the forces in play are far greater and it is a common experience to see the strong attraction a magnet exerts on a piece of iron, nickel, chrome and so on.

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