Low Voltage Excitation of the Spectrum of Caesium

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Abstract
Spectrum of caesium for excitation voltages below ionization potential.— While the Bohr theory suggests that individual lines should appear in regular sequence as the corresponding voltages are reached, previous attempts to observe them have failed. In this study a large equipotential source of electrons was secured by fitting a nickel cylinder 10 cm long over an alundum tube heated internally by a tungsten spiral. With this as cathode although currents up to 1 milliampere were used, the current density and also the space charge effects were small. With this tube heated so as to give a pressure of.01 mm both visual and photographic observations showed definitely that lines in the principal series of Cs appear in the order and within 0.1 volt of the potential required by the theory. Lines in the diffuse and sharp series were more difficult to observe, but they also appeared below the ionization potential 3.9 volts.

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