Contemporary Advances in Physics, XXI, Interception and Scattering of Electrons and Ions

M A G I N E a stream of electrons projected, all with known and uniform velocity and along the same direction, into a rarefied gas. Perhaps it ionizes the gas; if so, positive ions appear, and one may detect them and identify them and count them in any of various ways, without concerning oneself about the destiny of the ionizing electrons. Or perhaps it excites the gas without producing ions; if so, the atoms (or molecules) send forth light, and one may detect the excitation and identify the manner and measure the likelihood thereof, without paying any attention to the corpuscles responsible. Nevertheless, these corpuscles also must have been affected; they must have given up some at least of their kinetic energy, and if they still retain some motion, it is probably no longer in the same direction as at first. If there is ionization or excitation of the gas, there should be electrons wandering off obliquely from the stream, and moving more slowly than when they entered the gas; in technical language, there should occur "scattering with loss of energy." Even if the incoming corpuscles are moving too slowly to ionize or excite, there might be--and there are-- electrons wandering off obliquely with practically undiminished speed; they have suffered "elastic impacts" with atoms or molecules, they have been deflected merely, or "scattered without loss of energy." And even if the incoming corpuscles are moving fast enough to ionize or excite, some may be scattered with undiminished speed while others are spending some of their energy in these operations.