In accelerator physics strong focusing or alternating-gradient focusing is the principle that the net effect on a particle beam of charged particles passing through alternating field gradients is to make the beam converge. By contrast "Weak focusing" is the principle that nearby circles, described by charged particles moving in a uniform magnetic field, only intersect once per revolution.Earnshaw's theorem shows that simultaneous focussing in two directions at once is impossible. However, ridged poles of a cyclotron or two or more spaced quadrupole magnets (arranged in quadrature) alternately focus horizontally and vertically.