. The Bell System technical journal . ith great care. But letus first consider the direct experiments of the type which I have beenpresuming. The experiments would certainly be simplest, if the electrons weredashing along with enormous forward speeds, corresponding to visviva of the order of thousands of equivalent volts; for then their trans-verse speeds would probably be negligible, the beam would have little CONTEMPORARY ADVANCES IN PHYSICS 675 tendency to spread other than that for which the gas itself is account-able. As early as 1894, Lenard did such experiments with 30-kilovoltelectrons
. The Bell System technical journal . ith great care. But letus first consider the direct experiments of the type which I have beenpresuming. The experiments would certainly be simplest, if the electrons weredashing along with enormous forward speeds, corresponding to visviva of the order of thousands of equivalent volts; for then their trans-verse speeds would probably be negligible, the beam would have little CONTEMPORARY ADVANCES IN PHYSICS 675 tendency to spread other than that for which the gas itself is account-able. As early as 1894, Lenard did such experiments with 30-kilovoltelectrons (cathode-rays which had emerged from a discharge-tubethrough a window of thin metal foil). A few years later, he addeddata obtained with slower electrons, and the work was continuedby Becker and by Silbermann.^ For the 30-kilovolt corpuscles, thecross-section o- is very much smaller than the gas-kinetic co—only afew per cent as great. As the energy of the electrons is decreased, arises towards ao. AH this is illustrated in Fig. Apqioo 660 T T —r 500 1000 2000 4000 30000VOLT Fig. 1—Cross-section for interception of electrons, plotted for various gases over awide range of electron-speeds. (P. Lenard, Annalen der Physik.) Obviously, these are not experiments in which the corpuscles are un-able to ionize or to excite; quite the contrary. One might be temptedto rush to the other extreme, and guess that all of the electrons missingfrom the beam have effected ionizations, that the quantity a of Lenardsexperiments is a measure of likelihood of ionization; but in the presentstate of knowledge, this would be going too far. It is important,however, that for the/a5/ electrons a turns out to be proportional to theatomic number of the gas, if this is monatomic; and to the sum of theatomic numbers of the atoms constituting the molecule, if the gas isdiatomic or triatomic. This sort of rule suggests that interception offast-flying corpuscles is due either to the nuclei of the atoms,
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