A device that is used to "select" or "grab" particles from a particle source (such as a heated cathode or radioactive material) that are moving at a specific velocity.

This is useful because particles stream from a particle source at a plethora of different velocities.

There are two famous velocity selectors I've heard of. I'm sure there are many more ways to construct them, though.

The first kind of velocity selector is one you can use for charged particles. The Lorentz force on a charged particle is q(E+v x B). So lets say that you have a region which has perpendicular electric and magnetic fields. Now you can arrange the field strengths so that for a particular velocity, the electric and magnetic fields exactly cancel each other out. This velocity is obviously E/B where E and B refer to the magnitudes of the fields. Thus if particles of this velocity pass through this region they will pass through undeflected. Particles of any other velocity will be diverted. Note that the charge doesnt enter into the picture and neither does the mass. However the particles must be charged.
So this device acts like a velocity selector because if you send a beam of particles of varying velocities through it, only those particles of velocity E/B will come through straight. Everthing else gets deviated away.

The second kind of velocity selector was probably first used by Fizeau in his famous speed of light experiment. The idea here is much simpler and the particles dont need to be charged. you just have two toothed wheels, which have the same frequency of rotation. Particles pass through the spokes of the first wheel, but if any one of them has the wrong velocity, it hits the spokes of the second one and is stopped. Those which have the right velocity sail right through. Its a bit like traffic lights. If you have the right velocity you can go through all of them without ever getting stopped(so they say!!) but if you are going too fast or two slow you are bound to get stopped.
This is not what Fizeau used it for though. He used it to create a 'chopped beam' of light, but thats another story.

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