| Popis: |
Summary Experiments are described concerning the starting potential and the characteristic of the glow discharge between coaxial cylinders in an axial magnetic field at low gas pressures. With a magnetic field H of sufficient strength electrons, leaving the cathode, describe a cycloidal path of considerable length before reaching the anode. So the magnetic field causes an apparent increase of the pressure (up to a factor 1000) and a decrease of the starting potential Vz (down to a factor 0,01). (Fig. 6–9). At very low pressures p and large values of H it is to be expected that the electrons, liberated from the cathode, should return to the cathode before collision and should be useless for the starting mechanism. Nevertheless in this region in the tubes I and II (fig. 4), with the outer cylinder cathode, low starting potentials are obtained (fig. 9), which may be due to: roughness of the cathode surface, electron liberating from the curved end parts of the outer cylinder and ionising of gas atoms by positive ions. The curved end parts of the outer cylinder, when used as cathode, are essential for the decrease of Vz by the magnetic field; when the outer cylinder is taken as anode, Vz is much higher (fig. 10). This tube may therefore be used as a rectifier. Vz is also increased considerably, when the outer cathode cylinder is left open at both sides (fig. 15). The influence of the angle ♀, which the direction of the magnetic field makes with the axis of the cylinder, was studied. By increasing ♀ the number of electrons returning to the cathode is diminished (fig. 11), but also the number of ions formed by one electron. So the increase of ♀ causes sometimes a decrease (fig. 12) and sometimes an increase (fig. 13) of Vz. The peculiar form of the curves in fig. 13 is discussed. With an open outer cylinder as cathode the starting potential is lower in an inhomogeneous field (short coil around the middle of the tube) than in a homogeneous field (long solenoid), which can be explained by the form of the electron paths in both cases (fig. 14). In a certain region of values of p and H, Vz has more than one value (fig. 7–9), consequently the current-voltage characteristic has 2 intersection points with the V-axis. At a somewhat lower value of H characteristics were found, which did not intersect the V-axis at all (fig. 16, 17); a characteristic of this kind contains four different parts: besides an ordinary positive and negative region also a positive and a negative region with different properties. These latter regions were found previously in helium (without magnetic field) and called “characteristics of the second kind”. Points on a positive characteristic of the second kind cannot be measured as here the discharge cannot be stabilized by a series resistance. As the magnetic field acts as a apparent increase of the gas pressure, the current densities of the glow discharge at low pressure can be largely increased by it (In Fig. 18 with a factor 104). For pressures |
