With artificial willemite as the luminescent substance, it is found that, for sufficiently low current densities, J, and for accelerating potentials, V, between 200 and 800 volts, the brightness of luminescence, B, is given quite accurately by B=KJV2. This empirical equation gives too high values if J exceeds a limiting value which depends upon the accelerating potential, and which becomes lower as the potential is raised. From 800 to 1500 volts the agreement is only approximate, B increasing somewhat more rapidly than the equation indicates. There is no indication of the existence of a “threshold” voltage for this material.
The chief difficulty encountered in working at these low accelerating potentials is the uncertainty concerning the potential of the surface of the luminescent target, and one important aspect of the present work is the development of experimental means to determine the potential of this surface to within the limits of experimental error of the other measurements. It is shown that when the target is very carefully shielded electrostatically the secondary emission from it will maintain its potential within a very few volts negative to the potential of the shield, over the entire range of voltages used.
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