the minimum potential difference between electrodes in a gas required to produce a self-sustained discharge—that is, a discharge that can be sustained without any external ionization sources. A self-sustained discharge is maintained by ionization processes occurring in the gap between the electrodes and at the cathode; the intensity of the processes increases as the potential difference between electrodes increases. The firing potential is equal to the potential difference at which the intensity of ionization processes is sufficient so that each charged particle generates another, similar particle before its “disappearance.” The magnitude of the firing voltage is determined by the nature and the pres-sure/? of the gas, by the material, shape, and surface conditions of the electrodes, and by the interelectrode gap d. In a uniform electric field the firing voltage depends on the total number of gas atoms in the gap between the electrodes—that is, on the product pd (Paschen’s curves for several gases are shown in Figure 1).
Even insignificant impurities in* the basic gas with which the system is filled have a strong influence on the magnitude of firing voltage; such an influence is also exercised by formation of thin films of foreign atoms on the surface of the cathode. The action of external ionizing factors (for example, radioactive radiation) in the discharge gap or on the electrode surfaces causes a decrease of the firing voltage.