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Introduction |
ESPERIMENTI INTERATTIVI ED EXHIBIT
Francois Arago, France, 1786-1853 
And an apparatus dating back to the same period: Arago's Disk. On rotating, the copper disk on the right pulls the horseshoe magnet above it. The phenomenon remained a mystery until Faraday explained it some time around 1831, more than ten years later. 
For many years Arago worked on the determination of the meridian that passed through Paris, adopted prior to that of Greenwich. In Paris there are still these small brass plates (about 190 of them) fixed to the ground to mark "Arago's meridian". 
Arago and Ampere discuss electromagnetic phenomena... 
A magnetic needle (here it is a strip of tin with two small magnets at the ends) is suspended over a disk of aluminium, a non-magnetic metal. Let's see what happens.
This left the natural philosophers of the times dumbfounded for many years. As can easily be imagined, they experimented by replacing the disk with any material that came to mind, even beefsteaks, and in all cases obtained inexplicable results. From Faraday onwards there was no longer any mystery. The explanation lies in the currents induced in the disk by the magnetic field of the needle which generates in the disk a magnetic field that opposes the variation that generated it: see the following. 
And now another experiment which in a certain sense is complementary to the previous one:
A fundamental application that arrived more than 160 years later:
The rotating magnetic-field motor, invented by Galileo Ferraris: here we see a model:
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