What is it about?

This paper describes the unusual phenomenon of electromagnetic induction that occurs when a Nd-Fe-B magnet is passed through a hollow paramagnetic aluminum tube and an eddy current is conducted. It was observed that the time for the Nd-Fe-B magnet to pass through a 1 m aluminum tube is longer than that of a 2 m tube. A tentative theory for this phenomenon is that, due to the paramagnetism of the aluminum tube, the 2 m tube induces a more potent induction to pull the Nd-Fe-B magnet downward than that present for the 1 m tube. This scientific effect, which is coined herein as a length-boost effect, can create the situation that contradicts the conventional thinking that it should take a longer time to fall through a longer tube. However, diamagnetic tubes, such as those composed of copper and brass, do not possess this effect. A magnet falling through longer tubes of these metals takes a longer time than for the shorter tubes.

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Why is it important?

When dropping a magnet in a non-ferromagnetic conductor, such as a copper or aluminum tube, the magnet is braked by the magnetic field induced in the conductor, such that the magnet drops at a much slower speed than a free-falling one. However, does the speed of a magnet drop in a monotonically increasing way? Apparently, this should be so. However, our experiment shows that it is otherwise. The pattern of falling speed could vary according to whether the tube is paramagnetic or diamagnetic. One application for this is to control the velocity of a falling Nd-Fe-B magnet by changing the length of an aluminum pipe. The other is to consider using aluminum as braking fins in free fall chairs at amusement parks to reduce heat dissipation.


One reason for the length-boost effect is the paramagnetism of the aluminum pipe. The paramagnetism of the aluminum pipe aligns with the magnetic field in the magnet, whereas the diamagnetism of the copper pipe pushes itself away from the magnetic field in the magnet. The longer the aluminum pipe is, the greater the paramagnetism it creates, implying that more downward force is given to the magnet. This downward force coming from the paramagnetism of the aluminum pipe attenuates the anti-magnetic field (which is going upward). The three combined forces (downward force from paramagnetism, upward force from the anti-magnetic field, and downward force of gravity) make the magnet move faster when the aluminum pipe is longer.

Jyhjeng Deng
Dayeh University

Read the Original

This page is a summary of: The unusual phenomenon of electromagnetic induction, AIP Advances, November 2021, American Institute of Physics, DOI: 10.1063/5.0064760.
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