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The effect of magnetic fields on moving charged particles
A magnetic field will curve the trajectory of charged particles (more details...).
Positively charged particles are curved in one direction, negatively in
opposite direction (move cursor 
onto and off the picture).
Usually it is not necessary to be told which way the field is pointing - the picture already contains the answer. The little curly tracks are produced by electrons which are knocked out of the atoms by charged particles passing by.
Electrons (and anti-electrons) spiral because they are much lighter than all other charged
particles and lose energy quickly by another process called bremsstrahlung (more details ...).