ALMOST simultaneously with the appearance of Bohr's first paper on atomic structure, Franck and Hertz, developing an idea of Lenard's, inaugurated a series of experiments which were to prove of great importance to theories of atomic constitution. They undertook to study the effect of collisions between slow electrons and gaseous molecules as a function of the speed of the electrons and the nature of the gas. Other investigators, notably Horton and Davies in England, McLennan in Canada, and Compton, Foote and Mohler in this country, took up the work and soon its direct relation to Bohr's ideas became apparent. It would be an interesting study to trace the mutual reactions of the two lines of investigation through the past decade, the experimental and the theoretical, each helping to solve the other's problems, yet at the same time propounding new ones. Such a discussion, however, is beyond the scope of this paper, which is less concerned with the triumphs of these experimental methods than with their limitations. These can be made evident by a brief presentation of the principles involved and the results obtained up to I9 2I.
All these experiments on " ionization potentials " are done in gases at low pressures in tubes in which are sources of low speed electrons, usually filaments, and various grids and plates insulated from each other. The general idea is to increase the speeds of the electrons (whose initial speed is small) by applying a known electric field and to study the variation in the currents between electrodes as a function of the applied field. The methods are dis-