The behavior of a fine wire placed axially in a cylindrical anode is investigated.
A brief account is given of the more recent literature dealing with the subject.
A quartz tube which has been found particularly suitable and convenient is described. As it is not possible to outgas the ends of the filament and the supports, these parts have been shielded from the field.
The usual source of high potential consisting of transformer, rectifier and condensers is used.
A description is given of a method for measuring potentials from a few volts up to 25,000 volts. Use is made of a galvanometer and series resistance, and it is thought that the range can be extended to at least 100,000 volts.
With heat treated wires results are obtained in good agreement with earlier work, but it is shown that with such wires an appreciable current is obtained only after a break has occurred in the surface of the wire. The resulting characteristic depends only on the power available during the break and is shown to obey the empirical equation:
i=Ce-b/V
Microphotographs are presented which makes it plausible that the potential gradient may exceed 40 million volts/cm without producing currents in excess of 10-10 amperes and that the gradient is probably 108 volts/cm or more when reasonably large currents are obtained.
With untreated wires a "loop phenomenon" is described which is thought to be due to positive ions from the anode. Heating thetube in an electric furnace is shown to have a great influence on the emission in this case. Microphotographs show that the diameter of such a wire decreases with use.
With thoroughly outgased wires the emission is not changed when the tube is submerged in liquid air, and the heating of the filament up to 1400°K increases the emission only a few percent.
