Volume 8, Issue 11, 01 November 1937
Index of content:
8(1937); http://dx.doi.org/10.1063/1.1710250View Description Hide Description
8(1937); http://dx.doi.org/10.1063/1.1710253View Description Hide Description
An insulated surface uniformly bombarded with electrons of a given energy attains a potential relative to other elements in a high vacuum tube such that exactly as many electrons leave the surface by secondary emission as there are primary electrons arriving. A tube and circuit have been constructed for determining accurately this potential for willemite surfaces while being bombarded by electrons of energy range up to 10,000 volts. It was found that the surface potential was only slightly negative with respect to the most positive anode of the tube over the lower range of energy but finally reached an upper limit of potential relative to the cathode usually between 6000 and 7000 volts. The resistance between wires covered by the willemite was found to be independent of the bombarding current and voltage except in so far as this bombardment changed the temperature of the material. A special potassium phototube is described which was used to determine the light output from the bombarded surface as a function of the current density and the electron energy. The light was found to be accurately proportional to the current for densities below 2.5×10−6 amp. per sq. cm but above this a saturation effect set in quite gradually so that with a current of 20×10−6 amp. per sq. cm the light output was only 65 percent of the value which it would have been had the light intensity per unit current density remained constant. The light output for a constant current was found to increase with (V – V 0)2 where V 0 is the ``dead voltage'' when the bombarding energy was less than that required to penetrate through the individual grains of the willemite. An experimental nine‐inch cathode‐ray tube was also studied for screen potential and light output as a function of current density and anode voltage. The difference in potential between the screen and the anode was again small for the lower range in voltage but with anode voltages above 7000 volts, the apparent screen potential depended on the current density so that a change of potential of the screen of as much as 1500 volts was observed as the current density was increased from one to ten milliamperes per sq. cm. The light output increased with the square of the bombarding potential up to 1500 volts but above this it increased with the 1.2 power of the voltage. The light output per unit current density observed at 10−2 amp. per sq. cm was only two percent of that observed at 10−6 amp. per sq. cm on account of the severe ``saturation'' effect found at high current densities.
8(1937); http://dx.doi.org/10.1063/1.1710254View Description Hide Description
The present study of the cathode region in the glow discharge is divided into three sections: I. The range of the electrons in the negative glow. II. Positive ion formation in the Crookes dark space. III. Energy of the electron entering the negative glow. I. The length of the negative glow and the drop in potential across the Crookes dark space have been measured simultaneously for various gases over a wide current and pressure range. The observed lengths correspond exactly to the range of electrons as determined by Lehmann for voltages equivalent to the cathode potential drop. From this it is concluded that the energy of the electrons entering the negative glow must correspond to the entire difference in potential across the dark space. II. The length of the Crookes dark space has been measured under various conditions and the values obtained compared with the mean free path between ionizing collisions as given by Tate and Smith. The number of positive ions found in the dark space per electron of current has been computed by making use of Aston's equation for the potential distribution and Tate's values for the efficiency of ionization as a function of voltage. The number of positive ions so computed is, in general, materially less than one per electron; this contrasts sharply with from 50 to 100 estimated by some investigators. III. The energy distribution of the electron entering the negative glow has been measured by the deflection method, a nonhomogeneity of velocity in the electron beam being discernible by an elongation of the fluorescent spot on a Willemite screw. No change in the shape of the spot was detected for the electrons leaving the dark space although an elongation of it was observed for electrons in the negative glow. The experimental arrangement was such that fluctuations in voltage of more than 10 percent could have been observed. The general conclusion to be drawn from these studies is that the energy of the electrons entering the negative glow corresponds closely to the entire cathode potential drop, and that only a small fraction of the energy of the electrons leaving the cathode is expended in the Crookes dark space. A possible mechanism for the cathode region is briefly outlined.