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Mechanical Paul Trap

A spinning saddle traps a marble

In Short

A marble will always roll of a stationary saddle. However, when the saddle spins at just the right speed and the marble is placed at just the right spot it gets trapped.


A Skill Game showing how trapping a particle in a time-varying quadrupole potential works.


Details and Theory

Test your fine-tuning skills with this mechanical analogue of a Paul trap. This is a device normally used to capture charged particles. It is impossible to “trap” a particle with stationary electromagnetic fields. The only way to do this is by constant change. This is achieved by a rotating saddle potential. In the electromagnetic Paul trap, the potential comes from the electromagnetic fields; in this version, we use the Earth’s gravitational potential and a special saddle-shaped 3D printed sculpture. As the saddle rotates, a marble placed in the centre will remain relatively still, because when it starts to roll down one side, the saddle has already moved on. This will have changed the slope, so that the marble would have to roll ‘uphill’ and will therefore be forced to stay in the centre. The absolute values of the potential and the frequency of rotation determine which type of charged particles (mass-to-charge ratio) can be captured in the trap. By tuning the speed of the motor, you may be able to find a frequency at which the marble will stay in the centre – be careful, sometimes the marble will be knocked out of the saddle!