IF an electron is sent across a beam of electrons the chance is very small that it will pass a single electron of the beam so closely as to cause any deflexion of consequence from its original path. In a beam that has a current-density of 100 mA./cm.2 and a velocity of 100 electron-volts, the electron density is such that it contains 109 electrons per cm.3. So the mean distance between two neighbouring electrons may be said to be about 10â3 cm. To obtain a deflexion of 1°, however, a crossing electron would have to pass an electron of the beam at a distance of about 10â7 cm.