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Theory of the electric arc.-(i) Thermionic emission from the cathode. The fundamental phenomena of the arc are the cathode fall and the copious emission of electrons from the cathode. J. J. Thomson first suggested that this emission is chiefly of thermionic origin. But is thermionic emission sufficient to account for observed primary arc currents? Using the best data as to the emission from carbon and tungsten, the computed thermionic currents in the case of the carbon arc, the tungsten arc in hydrogen, and low-voltage, low-pressure arcs in various gases are found to be of the right order of magnitude. Brauer attempted to measure the thermionic emission directly by suddenly reducing the arc voltage to below the ionizing potential and obtained low values; but from a discussion of the effect of space charge on the current from the cathode it is shown that he certainly did not measure the thermionic emission but probably measured only the fraction released as a result of the effect on the space charge of positive thermionic emission from the anode. These facts and others all favor the thermionic theory as against the photo-electric and canal-ray theories of the origin of the electronic emission.(2) Current carried by positive ions, and cathode fall. If i is the current density of electronic emission, there must be a current density of positive ions j -2/242M 112 to neutralize the space charge due to i y where M is the atomic weight of the ions, and also an additional positive current density J to maintain the excess positive space charge. From theoretical considerations it is found that / = I.47(IO)-7 F C 3/2 X 1 / 2 /-M' 1/2^/2 , where V c is the cathode fall in volts, X the electronic mean free path,,and c is the cathode dark space.Incidentally, since c is approximately equal to X, J varies as the square of the pressure. An application of this expression to several cases, the carbon, hydrogen, and mercury arcs, leads to reasonable values for J. The ratio of positive to negative currents, (j + J)/i, is computed also from energy considerations at the cathode for various cases, with results that are consistent with the values obtained independently from the above equations. This is good evidence for the general correctness of the theory. (3) Ionization in the region between the electrodes of a carbon arc must be sufficient to neutralize the space charge due to the electrons. Reasons are given for concluding that neither emission from the anode nor ionization by collision will account for this ionization but that it is primarily of thermal origin. An expression for the current is derived and it is found that if the temperature is 4000 0 K or over, and if the ionizing potential is about 8.6 volts or less, the thermal ionization estimated by applying Nernst's Heat Theorem, alone is sufficient. Photo-electric ionization if present would be secondary, a result of the radiation accompanying recombination. (4) The anode fall is accounted for in a qualitative way by a deficiency of positive ions near the anode due to decreas...

The developments in nuclear physics emphasize the need of a new technique adapted to deliver enormous energies in concentrated form in order to penetrate or disrupt atomic nuclei. This may be achieved by a generator of current at very high voltage. Economy, freedom from the inherent defects of an impulsive, alternating or rippling source and the logic of simplicity point to an electrostatic generator as a suitable tool for this technique. Any such generator needs a conducting terminal, its insulating support and a means for conveying electricity to the terminal. These needs are naturally met by a hollow metal sphere supported on an insulator and charged by a belt conveying electricity from earth potential and depositing it within the interior of the sphere. Four models of such a generator are described, three being successive developments of generators operating in air, and designed respectively for 80,000, 1,500,000 and 10,000,000 volts, and the fourth being an essentially similar generator operating in a highly evacuated tank. Methods are described for depositing electric charge on the belts either by external or by self-excitation. The upper limit to the attainable voltage is set by the breakdown strength of the insulating medium surrounding the sphere, and by its size. The upper limit to the current is set by the rate at which belt area enters the sphere, carrying a surface density of charge whose upper limit is that which causes a breakdown field in the surrounding medium, e.g., 30,000 volts per cm if the medium is air at atmospheric pressure. The voltage and the current each vary as the breakdown strength of the surrounding medium and the power output as its square. Also the voltage, current and power vary respectively as the 1st, 2nd and 3rd powers of the linear dimensions.

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