Professor Maxwell's Duplicitous Demon by Brian Clegg

Author:Brian Clegg [Brian Clegg]
Language: eng
Format: epub
ISBN: 9781785784965
Publisher: Icon Books Ltd
Published: 2019-06-14T16:00:00+00:00

Maxwell’s electromagnetic spheres

Maxwell’s first attempt with his mechanical model was to imagine that the magnetic field was made up of a collection of spheres,‡‡ tightly packed to fill space. These spheres or ‘cells’ would be spinning around. Generally speaking, when a physical object spins, the centrifugal forces§§ acting on it make it spread out in the middle and contract at the poles. This happens, for example, to the Earth, which it had been known since Newton’s time has a bulge around the equator and so is an oblate spheroid rather than a perfect sphere.

But, unlike the Earth, Maxwell’s spheres were surrounded by other spheres. So, if the equator of a sphere expanded as it was spinning, it would push on the surrounding spheres. In this model, the axes of the spins were aligned to the lines of force that Faraday had demonstrated in the magnetic field. The result would be very close to what was observed. At right angles to the lines of force – the direction of the equators – the forcing outwards of the spheres would produce a repulsive effect, while along the lines of force – the direction of the poles – the spheres would be pushed closer together and the effect would be an attraction.

Conveniently, the faster the spheres rotated, the bigger this effect would be – so the spin rate in the model corresponded to the strength of the magnetic field. In this kind of mechanical model, it’s perfectly possible for the components to be allowed to be frictionless, but Maxwell thought it better to allow for a degree of interaction between the spheres. If two spheres alongside each other are turning in the same direction, then at the point of contact, the surfaces will be moving in opposite directions.

If you imagine two spheres, moving clockwise, with their axes pointing out of the page, the left sphere’s surface moves down at the point of contact while the right sphere’s surface moves upwards (Figure 3).

To avoid direct interaction between the spheres, Maxwell imagined a large number of much smaller spheres acting like ball bearings between the main spheres. But unlike the ball bearings in a traditional device, which are usually constrained by a bearing, these would be free to flow as they like. And if these little spheres were considered as particles of electricity (what we’d now call electrons), when an electrical circuit was made, the little spheres would flow in the channels between the bigger ones. (Let’s call the bigger spheres cells, as Maxwell did, to avoid getting our assorted spheres confused.)

FIG. 3. Two spheres rotating in contact.

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