In order to extend the low-pressure limit of the hot cathode ionization gauge, it is necessary to increase the ratio of ion current to X-ray photocurrent. This ratio may be increased by increasing the sensitivity of the ionization gauge. Therefore, the emitted electrons from the cathode have to travel in long paths before they are collected by the positive grid or anode. The two types of the hot cathode magnetron gauge, the Lafferty type and the coaxial type were fabricated and their characteristics were measured. Under normal operating conditions, these magnetron gauges are operated in a magnetic field with an intensity of 2. 5 times stronger than the cutoff value for the Lafferty gauge and 2 times stronger than the cutoff value for the coaxial gauge. A very low level of electron emission is used to obtain a stable operation in accordance with Lafferty's suggestion. The characteristics of the both types of the gauges are compared and discussed.
As the results, the ion current in the coaxial gauge is more sharply risen up to a flattening region than in the Lafferty gauge. For the characteristics of the gas ionization probability by electron impact, the position of maximum ion current in the coaxial gauge is shifted to lower anode voltage than in the Lafferty gauge. Other characteristics of the both types of gauges have been essentially the same in operation.
The positive ion emission from the cathode is suppressed by a careful construction of the filament on the axis of the cylindrical anode for the both magnetron gauges.
The measured sensitivity and the X-ray photocurrent indicated that the linear relationship between the pressure and the ion current may be established. The ion currents of magnetron gauges for Lafferty type and coaxial type have decreased linearly with a pressure ranging from 5 × 10^-6 to 10^-12 torr and from 1×10^-6 to 10^-12 torr, respectively. The measured pumping speeds of these gauges were found to be one-tenth as large as that of the conventional ionization gauge under normal operating conditions.

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