The total energy to be transferred to the system in order to remove the electron to an infinite distance from the nucleus is equal to the kinetic energy of the bound electron. If, therefore, the electron is removed to a great distance from the nucleus by impact of another rapidly moving electron, the smallest kinetic energy possessed by the latter when at a great distance from the nucleus must necessarily be equal to the kinetic energy of the bound electron before the collision. The velocity of the free electron therefore must be at least equal to
.
According to Whiddington’s experiments[36] the velocity of cathode rays just able to produce the characteristic Röntgen radiation of the so-called
-type—the hardest type of radiation observed—from an element of atomic weight
is for elements from Al to Se approximately equal to
