A thought experiment about Heisenberg's Uncertainty Principle If there is a box full of magnetized particles which are taking random movement continuously, then I put a coil connected to a device that can detect the value of current in it and it's designed to only allow one particle to pass(the coil is like digging a hole in a metal plate). Now when a particle passes through this coil, I can know it's velocity by calculating the current, and I know where the coil is, so I have exact velocity and location of this particle, then what's the problem?
 A: There is a basic misunderstanding of the Heisenberg Uncertainty Principle here.
$$\sigma_x\sigma_p\geq \frac{\hslash}{2}$$
It is an inequality on the product of two measurements. 
One can do a measurement on  a particle in a magnetic field and get the momentum and there are many $x,y,z$ points in the picture. 


Pi mu e decay in bubble chamber

The HUP does not say the measurements cannot be done. It just predicts  a limit to the product of the two measurements, by the very small number $\hslash\;.$
In the picture the big circle is a pion (identified by ionisation) turning in  a magnetic field perpendicular to the picture. There are two views of the event and both momentum and all the $x,y,z$ of the circle are known. The HUP is fulfilled because the space errors are in microns , the momentum is in $\rm{MeV}$ and there is no problem in fulfilling the HUP, as $\hslash$ is $\thicksim 6.582\times 10^{-16}~\mathrm{eVs}.$
The limitations of the HUP enter in specific measurements where the measurement errors are very small. It says that because of the quantum mechanical uncertainty the momentum and position measurement will have a delta spread due to the HUP, even though  the experimental measurement errors may be very small.
