The Photoelectric Current

Author: J.B. Hoag

The electric current, as measured by the meter G of Fig. 20 B, is directly proportional to the number of photoelectrons emitted from the cathode each second. This number depends, for a given cathode material, upon the intensity and the color of the incident light. Let I stand for the intensity of a beam of light of but one color (monochromatic) which falls upon the cathode. It is found that the number of photoelectrons is directly proportional to I.

If the intensity of the light is doubled, the photo-current i is likewise doubled, etc. This is expressed mathematically by the equation:

i = S · I, (λ constant),

where S is a constant. Figure 20 C tells the same story in a graphical way.

Fig. 20 C. The photoemission i is directly proportional to the intensity of the light, I

This simple relationship between the photoelectric current and the incident light is accurate over an extremely wide range of intensities, from 0.000,07 to 10,000 foot-candles; that is, from light intensities less than the eye can detect up to direct sunlight. It is accurate for thick and for thin coatings of various materials on the cathode or sensitive surface. It is also exact when the incident light is made up of a combination of many colors, provided only that the relative distribution of energies in the various colors remains unchanged as the intensity is changed. Departures from this simple law, which occur in practical photoelectric cells, are due to the accumulation of layers of negative electricity on the walls of the enclosing glass bulb. In the modern photoelectric tubes the effect of these changes is greatly reduced by proper design and position of the electrodes.