1. The method generally employed in investigating the angular scattering of electrons in gases consists essentially of firing a directed beam of electrons into a “field-free” enclosure, containing the gas to be investigated at a pressure of a few hundredths or thousandths of a millimetre. The electrons in the beam will be scattered, on colliding with the gas molecules, through various angles. Those which are scattered between θ and θ + dθ are selected by means of a pair of slits which can be set at any desired angle to the electron beam. The electrons selected by these slits are then resolved by means of retarding potentials or electric or magnetic fields into those which have been elastically scattered and those which, on being scattered by the molecule, have excited or ionised it. By plotting the final current received by the Faraday cylinder against θ, we can obtain the angular scattering curve for the elastically or inelastically reflected electrons.