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I remember years ago in school my chemistry teacher showed us a tangled wire that untangled itself when a current is applied, can anyone suggest what the material may be?

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  • $\begingroup$ Furthering this, what about electro-active shape memory polymers, are they a heat free alternative of shape memory alloys? $\endgroup$ – Agrif123 May 29 '18 at 8:30
  • $\begingroup$ Relevant video: youtu.be/FBaIdvgbBAM $\endgroup$ – Nilay Ghosh May 30 '18 at 9:20
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You are almost certainly thinking of nitinol wire or "memory wire".

However, it's not electricity that makes it untangle. It's heat. Running current through the wire is just a way to heat it.

When at room temperature, nitinol wire can be easily bent. When heated, it acts like a spring trying to go back to its unbent shape. You can see the same effect by twisting some wire, then dropping it into boiling water.

There have been "electric pistons" built on this principle. The piston is driven by a spring of memory wire. When cold, the piston is easily compressed. When the spring is heated by running electric current thru it, it pushes against the piston harder than what it took to push the piston in when cold. This effect has niche uses but is mostly a curiosity. The overall cycle is not very efficient.

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  • $\begingroup$ Oh I understand, thanks for your response. So I’m assuming relatively large currents are necessary for reaching the temperatures required for it to return to its original shape? $\endgroup$ – Agrif123 May 28 '18 at 18:32
  • $\begingroup$ @Agrif123: Metals generally have very low specific heat, so probably not. You realize an incandescent bulb filament reaches 3000°C almost immediately after you switch it on, right? $\endgroup$ – R.. GitHub STOP HELPING ICE May 29 '18 at 0:12
  • $\begingroup$ Out of curiousity: I guess your "electric pistons" get the energy to "push harder" from the heat. But don't the laws of thermodynamics tell us that you cannot convert heat directly into energy? $\endgroup$ – allo May 29 '18 at 8:22
  • $\begingroup$ What I find most intresting are nitinol implants that fold into their form when heatet by body temperature, like stents. $\endgroup$ – PlasmaHH May 29 '18 at 9:09
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    $\begingroup$ @allo You can get useful energy out of a temperature difference/gradient. The piston would not work if the whole engine was hot, but it can work when the wire is hotter than the surroundings. Likewise, the pistons in a car engine wouldn't work if the surrounding atmosphere was as hot as burning petrol, but they can work when the surroundings are cool. You can also "power" an engine using "cold", e.g. using a block of ice to run a Sterling Engine: it's the difference which matters. $\endgroup$ – Warbo May 29 '18 at 10:38
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That was a shape memory alloy, probably nitinol.

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