One of the most vital assertions made in Einstein’s theory is that the kinetic energy with which monochromatic light ejects electrons from any metal is proportional to the frequency of the light, i.e., if violet light is of half the wave-length of red light, then the violet light should throw out the electron with twice the energy imparted to it by the red light. In order to test whether any such linear relation exists between the energy of the escaping electron and the light which throws it out it was necessary to use as wide a range of frequencies as possible. This made it necessary to use the alkali metals, sodium, potassium, and lithium, for electrons are thrown from the ordinary metals only by ultra-violet light, while the alkali metals respond in this way to any waves shorter than those of the red, that is, they [Pg 240] respond throughout practically the whole visible spectrum as well as the ultra-violet spectrum. Cast cylinders of these metals were therefore placed on the wheel (Fig. 33) and fresh clean surfaces were obtained by cutting shavings from each metal in an excellent vacuum with the aid of the knife , which was operated by an electromagnet outside the tube. Fig. 32 After this the freshly cut surface was turned around by another electromagnet until it was opposite the point of Fig. 33 and a beam of monochromatic light from a spectrometer was let in through and allowed to fall on the new surface. The energy of the electrons ejected by it was measured by applying to the surface a positive potential just strong enough to prevent any of the discharged electrons from reaching the gauze cylinder opposite (shown in dotted [Pg 241] lines) and thus communicating an observable negative charge to the quadrant electrometer which was attached to this gauze cylinder. Fig. 33 For a complete test of the equation it was necessary also to measure the contact-electromotive force between the new surface and a [Pg 242] test plate . This was done by another electromagnetic device shown in Fig. 32, but for further details the original paper may be consulted.[168] Suffice it here to say that Einstein’s equation demands a linear relation between the applied positive volts and the frequency of the light, and it also demands that the slope of this line should be exactly equal to . Hence from this slope, since is known, it should be possible to obtain . How perfect a linear relation is found may be seen from Fig. 34, which also shows that from the slope of this line is found to be , which is as close to the value obtained by Planck from the radiation laws as is to be expected from the accuracy with which the experiments in radiation can be made. The most reliable value of obtained from a consideration of the whole of this work is