Il Museo di Fisica di Sardegna

	Interactive experiments and exhibits News Events F G C
	Introduzione Arago's disk Oersted's experiment Electromagnetic induction The Foucault current Electrodynamic actions Coils, solenoids and other things Measurement of a magnetic field Magnetic properties of materials Levitation experiment And if there's no variation in flow? The rotating magnetic field High frequency fields Photos	INTERACTIVE EXPERIMENTS AND EXHIBITS AND IF THERE’S NO VARIATION IN FLOW? Peter Barlow (1776-1862), English mathematician La Ruota di Barlow (1822) The structure is exactly the same as that of disco di Faraday with the difference that the wheel is a cogwheel, with the cogs that are immersed in a basin of mercury and are in the magnetic field of a horseshoe magnet. If a current is sent between the axis and the mercury basin the wheel begins to turn. Faraday’s disk and Barlow’s wheel are complementary, which is to say reversible, machines. This is perhaps the first “electric motor” built by humankind, but in any case twenty-three years after the voltaic pile! STRANGE GENERATORS AND STRANGE MOTORS WITH NO VARIATION IN FLOW Observe the upper part of the apparatus below. It is a copper disk with a contact on the axis and a ring magnet above it. To see what happens click on the photo. In this apparatus there is evidently no variation in flow linked to the conductor and thus Faraday’s Law would appear not to be applicable. What’s the explanation of what we have seen? (We suggest interpreting the facts by having recourse to Lorentz's Force). The apparatus below has exactly the same structure as the generator we saw above. The conductor disk and the ring magnet are integral with each other. The contact on the periphery draws from a drop of mercury in the groove on the periphery of the disk through the large curved conductor. To see it working click on the photo This is a strange direct-current motor: no brushes, no stator, with the electrical part and the magnetic part rotating together. And here too there is no variation in flow. And now the complement of the motor seen above. Here the disk and the ring magnet below it are fixed: it is the conductor that represents the contact at the edge of the disk which is free to rotate. To see it working click on the photo In reality this is a classic experiment, previously created by Faraday, known as the rotation of a current around a magnetic pole. The machines just seen belong to the category of so-called unipolar machines. An extreme case is that of a cylindrical magnet conductor polarized axially with a contact on the axis and another on the surface. When current is applied it begins to rotate. The oddity of such devices and the different interpretation of their behaviour, depending on whether the system of reference of the rotating part or the laboratory is used, suggested to Albert Einstein his first reflections on relativity, contained in the first paragraph of his work entitled "On the Electrodynamics of Moving Bodies" (1905), the founding act of restricted relativity. And finally, of strange motors, here is the strangest of all! Here we have no coils, no magnetic fields, no brushes but just two ball bearings connected by a conducting axis. To see it working click on the photo It turns quite well. The power is between the two outer rings. The explanation of its working is controversial. Is it electromagnetic? Some clues: 1. It works with both direct and alternating current; 2. it turns in either direction; 3. it needs a push to start it.