http://www.physics.orst.edu/node/475

hope this helps

Sunlight falling on a needle heats it up, which heats the air around it, which rises and induces an upward convection current past the needle.

The needle swinging through this weak upward convection current might have its motion slowed more quickly than in still air.

The Navier-Stokes equations were only derived in 1822, so this explanation might not have been considered at the time.

Alas, i don’t have the tools to simulate properly or run an experiment, but if you’ve got access to a finite element zealot, they could probably provide some insight.

My intention was not to belittle Maxwell’s contributions to electromagnetic theory, and I don’t think I stated that others had concluded that light was a ‘purely electromagnetic phenomenon’!

They were suspecting, however, that light must possess magnetic properties. Faraday especially had this view, as he notes in his introduction to his Faraday rotation paper:

I have long held an opinion, almost amounting to conviction, in common I believe with many other lovers of natural knowledge, that the various forms under which the forces of matter are made manifest have one common origin; or, in other words, are so directly related and mutually dependent, that they are convertible, as it were, into one another, and possess equivalents of power in their action… This strong persuasion extended to the powers of light, and led, on a former occasion, to many exertions, having for their object the discovery of the direct relation of light and electricity, and their mutual action in bodies subject jointly to their power; but the results were negative and were afterwards confirmed, in that respect, by Wartmann.

I’ll be writing up Faraday rotation in the near future, but it is unambiguous that Faraday was convinced of a direct relationship between light and electricity/magnetism. (And I haven’t yet found Wartmann’s paper!)

The other authors I refer to seem to have also been convinced that their work demonstrated more than a material relationship, but actual magnetic powers of light. From Christie:

The repeated failures of MORICHINI’S experiment of magnetising a needle by the violet ray, even under the most favourable circumstances, and in the ablest hands, have led many to doubt whether the effects, which were in some cases observed, were to be attributed to the influence of the ray ; but as the experiments which I have detailed indicate magnetic influence in the compound solar rays, and are besides easily repeated, they will, I think, tend considerably to remove these doubts.

Morrichini was more confused, and distinguished between the light and magnetism of the ‘solar rays’:

Terrestrial bodies may absorb from the solar rays the magnetic fluid as they absorb light and caloric.

“The real proof for me, however, is the huge time lag (more than 20 years) between Maxwell’s claim and Hertz’s experiment. ”

I’d be curious to know, though, if others had attempted experiments and failed before Hertz. Also, keep in mind that long delays were common back then. As an example, I note that Morrichini’s experiments were apparently done pre-1814, but in 1826 the controversy over the work was still going enough to get Somerville into print, and in 1845 the questions raised by them were apparently still not resolved!

I’d be careful to separate what is said casually in an intro course about Maxwell’s role and what is obvious from reading his paper. Maxwell invents something entirely new, literally out of thin aether, that completes the unification of electricity and magnetism that grows from the discoveries of Faraday and Oersted. He uses a post-prediction of Faraday rotation to provide additional support (beyond the, possibly coincidental, speed of light) for his conclusion.

But I don’t see anything in observations that light might interact with magnetic materials to indicate that people were concluding that light itself was a purely electromagnetic phenomenon. I mean, people were still arguing whether it was a corpuscle or a wave while this was going on. Even Faraday, known for his brilliant, non-mathematical leaps of intuition did not go that far.

The real proof for me, however, is the huge time lag (more than 20 years) between Maxwell’s claim and Hertz’s experiment. It’s not like the whole world was so confident in this discovery that they all went to work trying to produce Maxwell’s waves!

Modern physics eschews discovery through experimentation for observation based upon theory. It has rendered itself incapable of seeing anything unexpected (felonious negligent discovery – anything outside business plan, budget spreadsheet, or PERT chart is insubordination).

Before I go off course… About the observation of the Sun’s light effect on the magnetic needle: the Sun’s UV rays might photo-induce electric currents on the metallic needle. These currents may then circulate in the finite-size metallic needle thus inducing a weak magnetic dipole moment which “helps” damp the vibrations of the needle so as to align it with the Earth’ magnetic field… this is just an idea.