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The National MagLab is funded by the National Science Foundation and the State of Florida.

Contracting Helix

This device demonstrates how parallel wires attract because of the magnetic fields they generate.

A current-carrying wire generates around it a circular magnetic field in a way described by the right hand rule. That is to say, if you had a wire pointing from the bottom to the top of this page, the magnetic field to the left of that wire would be coming out toward you, while on the right side of the wire that circling field would be heading into the page.

As a result, when you put two such parallel wires together, with current traveling in the same direction, those wires will attract. At the point at which their respective magnetic fields intersect, they are traveling in opposite directions, and opposites attract.

English doctor Peter Mark Roget devised a contraption in 1835 that exhibited this principle in action. Known both as a contracting helix and Roget’s spiral, the device is a type of primitive motor, akin to Faraday’s motor.

Take a look at it here:


  1. Observe the wire helix hanging so that its bottom tip dips into a metal-bottomed glass cup containing mercury, a liquid which readily conducts current.
  2. Open or close the knife switch to see what happens when the helix is connected to current from the battery.
  3. The long series of parallel wires in the helix attract each other, causing the helix to contract. As it does, the end is lifted out of the mercury, breaking the circuit, stopping the current, and relaxing the spiral.
  4. As the spiral relaxes, the helix falls back into the mercury, re-connecting the mercury and starting the cycle again.
  5. You can also adjust the coil height to connect or disconnect the circuit.