Photoelectric Currents and the Battery Voltage
Author: J.B. Hoag
|Fig. 20 D. The photoelectric currents for different voltages applied to a vacuum (V) and a gas-filled tube (G). (From E. & N. P.)|
Figure 20 D shows two curves which are identical at the left but differ at the right. The one marked V is for the case when the photoelectric cell is very highly evacuated. The other, marked G, is for the case when there is a trace of gas in the bulb. The photoelectric current is plotted vertically and the voltage of the battery is plotted horizontally. In both of these curves it is assumed that the intensity of light and its color are constant.
When the cathode or sensitive surface is only slightly negative, and the anode slightly positive, the currents are small. As the battery voltage is increased, the currents quickly rise to a saturation value indicated by the long horizontal portion of the V curve. In other words, in a highly evacuated photoelectric cell, if the battery voltage is greater than a certain small value — shall we say 10 volts — all of the electrons which the light releases from the cathode will be drawn over to the anode. Then any change in the battery voltage cannot change the amount of the electrical current because all emitted electrons are in use. The additional current, shown in the curve G, is obtained from the gas in the tube. Electrons coming from the cathode, when sufficiently speeded up by the battery voltage, are able to eject electrons from the neutral gas atoms. The newly created electrons add their number to the original photoelectrons. In addition, the "ionized" atom, which is positively charged, moves toward the cathode. The net result of all of this is that the total current is increased. Furthermore, it is to be seen from the curve G that it increases very rapidly as the voltage on the cell speeds up the photoelectrons and increases the effectiveness with which they can ionize neutral gas atoms. The battery voltage which can be safely applied to a gas-filled cell, indicated roughly by the dotted arrow, lies between 75 and 125 volts. It is usually about 90 volts. Higher voltages may be applied when the intensity of the light is small, and vice versa. If greater voltages are used, the positively charged atoms will bombard the cathode too vigorously and destroy its ability to emit photoelectrons copiously.