. Radio-activity . s travel to the negative electrodeand the negative to the positive. Since the velocity of the ions between the plates is directlyproportional to the strength of the electric field, in a weak fieldthe ions take so long to travel between the electrodes that most ofthem recombine on the way. The current observed is consequently small. With increase ofthe voltage there is an increase of speed of the ions and a smallernumber recombine. The current consequently increases, and willreach a maximum value when the electric field is sufficientlystrong to remove all the ions before appr
. Radio-activity . s travel to the negative electrodeand the negative to the positive. Since the velocity of the ions between the plates is directlyproportional to the strength of the electric field, in a weak fieldthe ions take so long to travel between the electrodes that most ofthem recombine on the way. The current observed is consequently small. With increase ofthe voltage there is an increase of speed of the ions and a smallernumber recombine. The current consequently increases, and willreach a maximum value when the electric field is sufficientlystrong to remove all the ions before appreciable recombination hasoccurred. The value of the current will then remain constant eventhough the voltage is largely increased. This maximum current will be called the saturation1 current,and the value of the potential difference required to give thismaximum current, the saturation The general shape of the current-voltage curve is shown inFig. 2, where the ordinates represent current and the 1 This nomenclature has arisen from the similarity of the shape of the current-voltage curves to the magnetization curves for iron. Since, on the ionizationtheory, the maximum current is a result of the removal of all the ions from the gas,before recombination occurs, the terms are not very suitable. They have howevernow come into general use and will be retained throughout this work. n] IONIZATION THEORY OF GASES 31 Although the variation of the current with voltage dependsonly on the velocity of the ions and their rate of recombination,the full mathematical analysis is intricate, and the equations,expressing the relation between current and voltage, are onlyintegrable for the case of uniform ionization. The question is com-plicated by the inequality in the velocity of the ions and by thedisturbance of the potential gradient between the plates by themovement of the ions. J. J. Thomson1 has worked out the casefor uniform production of ions between two parallel plate
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