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THE GROWTH AND DECAY OF PHOTO-THERMIONIC CURRENTS FROM OXIDE COATED FILAMENTS
BY H. D. ARNOLD AND HERBERT E. IVES
RESEARCH LABORATORIES OF THE AMERICAN TELEPHONE & TELEGRAPH COMPANY AND WESTERN ELECTRIC COMPANY, INC.
Communicated by F. B. Jewett, October 18, 1921
T. W. Case1 and E. Merritt have recently described experiments in which the oxide coated filament of an audion is subjected to illumination, with the result of increasing the current between filament and plate. In the experiments of Prof. Merritt this added space current was shown to be principally due to short wave-length radiation, and was explained as a true photo-electric effect, that is, the direct emission of electrons. due to illumination. He found the added current to be dependent upon the filament temperature. This might be taken to indicate that the emission of photo-electrons is a function of the temperature of the surface, which disagrees with conclusions drawn from experiments upon other photo-electrically active materials.
In some recent experiments we have found this effect to have characteristics sufficiently different from those of the true photo-electric effect to raise doubts as to the identity of the two phenomena. If they are not identical then the new effect is not necessarily evidence for a dependence of the photo-electric effect on temperature.
The experiments here reported have to do with the rate of growth and decay of the light induced additions to the space currents. These were recorded on a string galvanometer through a distortionless amplifier system. In every case voltages were high enough to carry across all electrons liberated. A carbon arc lamp with glass condensing lens was used as a light source, and between the condenser and the oxide coated filament was interposed either a deep red or a deep blue glass (transmitting
no red), selected so that the light transmitted by each gave added space currents of the same order of magnitude.
With the deep red glass interposed the added space currents obtained upon brief illumination with various heating currents through the filament are shown in figure 1. The effect increases regularly with temperature, but takes a considerable time to rise and fall at the beginning and end of the period of illumination. The times of growth and decay depend upon the magnitude of the current, and the curves have the general appearance of heating and cooling curves. We believe this red light effect to be chiefly due to the heating of the filament by the incident radiation. Support of this belief is given by figure 2, in which the same increase of space current is caused, first (a) by red light, and second (b) by a sudden brief increase of filament heating current. It will be seen that the two curves are similar. We have also found that the added space current varies with filament current in the way that it would for equal added filament-power increments; and that the change of resistance of the filament is approximately the same whether the space current increment. is due to red light or filament current.
With the blue glass, the energy transmission, as measured by a thermocouple, was only a few per cent that of the red glass, although for a chosen filament temperature near the middle of the available range both transmitted radiations gave the same added space current. The heating effect of the blue radiation is therefore negligible, and the added space current may be considered as a true light effect. String galvanometer records of the added space current due to brief blue illumination, for various filament currents (temperatures) are reproduced in figure 3. It will be seen at once that the added space currents exhibit a behavior essentially different from those due to red light or heating. Whereas with red light the current continually increases with temperature, with blue light the added current at first increases and then starts to decrease. At low temperatures the growth and decay are exceedingly slow. the temperature is raised the rates of growth and decay increase until at the highest temperature studied the response appears practically instantaneous.
It is the existence of this slow response and its variation with temperature that we believe differentiate this effect from the true photo-electric effect, which, as far as previously known (and in agreement with tests made by us on a potassium cell) is instantaneous at all temperatures.
The true nature of the increased electronic emission under blue illumination is not decided by our experiments, but several possibilities suggest themselves. It may be a light induced increase in the number of electrons available for thermionic emission; or it may be a light induced change
Added space currents due to illumination of audion filament by red light. (a) Filament heating current
Equal added space currents caused by (a) illumination by red light; (b) increase of heating current.