Reversible permanent magnet
A reversible permanent magnet is a magnet that can be turned on and off and can keep either state without external power.
The video begins with the magnet in “on” mode, that is, it is holding the bar on the bottom against gravity. Note that the power supply is off (as seen in the background).
A charge pulse is applied (by switching the DC power supply on then off quickly) which changes the magnet to “off”. The bar on the bottom drops onto the table.
We hold the magnet near the bar and it is not attracted to the magnet. (This could have been done more dramatically in the video).
Next, the polarity of the DC signal is reversed (we reach behind and swap the banana connectors) and again a short DC pulse is applied. The magnet picks up the bar off the table.
Although we don’t explicitly show it clearly in the video, the power supply is off with the exception of the short pulses that turn the magnet on or off. The rest of the time the magnet stays in whatever state it is in; that is why it is still called a “permanent magnet”. This is also how this differs from a solenoid where the electrical input power must be provided any time the magnetic field is on.
Built by Ara Knaian, video by Amy Sun, both of the MIT Center for Bits and Atoms.