The photo-electric action of X-rays

Abstract

The fundamental law of photo-electric activity states that ½ mv 2 = hv - w , (1) where the left-hand side represents the maximum kinetic energy of the liberated electrons, h is Planck’s constant, v is the frequency of the exciting light, and w is a constant which measures the work necessary for an electron to escape from the substance, and whose value is characteristic of the material under consideration. The equation (1) was first given by Einstein as a deduction from the view that the energy of radiation was distributed in discrete quanta. However, I succeeded in showing that it followed from Planck’s radiation formula; so that it evidently has a wider basis than the restricted and doubtful hypothesis used by Einstein. On the experimental side the evidence for several years was somewhat conflicting, but in 1912 I showed, as a result of experiments made in collaboration with Dr. K. T. Compton, that the equation represented the photo-electric behaviour in the visible and in the ultra-violet as far as λ= 2 x 10 -5 cm., for the metals sodium, aluminium, magnesium, zinc, tin, and platinum as accurately as it could be determined. The experiments also showed that the differences in the values of the constants w for different metals were connected with the corresponding contact potentials V by the relation w n - w m = e (V n -V m ). (2)