In a weak electric field, the negative ions only produce ions by collision. The positive ion, whose mass is at least 1000 times greater than the electron, does not acquire a sufficient velocity to generate ions by collision until an electric field is applied nearly sufficient to cause a spark through the gas. An estimate of the energy required for the production of an ion by X rays has been made by Rutherford and McClung. The energy of the rays was measured by their heating effect, and the total number of ions produced determined. On the assumption that _all_ the energy of the rays is used up in producing ions, it was found that _V_ = 175 volts—a value considerably greater than that observed by Townsend from data of ionization by collision. The ionization in the two cases, however, is produced under very different conditions, and it is impossible to estimate how much of the energy of the rays is dissipated in the form of heat. =42.= Variations are found in the saturation current through gases, exposed to the radiations from active bodies, when the pressure and nature of the gas and the distance between the electrodes are varied. Some cases which are of special importance in measurements will now be considered. With unscreened active material the ionization of the gas is, to a large extent, due to the α rays, which are absorbed in their passage through a few centimetres of air. In consequence of this rapid absorption, the ionization decreases rapidly from the surface of the active body, and this gives rise to conductivity phenomena different in character from those observed with Röntgen rays, where the ionization is in most cases uniform. =43. Variation of the current with distance between the plates.= It has been found experimentally[74] that the intensity of the ionization, due to a large plane surface of active matter, falls off approximately in an exponential law with the distance from the plate. On the assumption that the rate of production of ions at any point is a measure of the intensity _I_ of the radiation, the value of _I_ at that point is given by $$ \frac {i}{i₀} = 1 − e^{–λ x} $$ where λ is a constant, _x_ the distance from the plate, and _I₀_ the intensity of the radiation at the surface of the plate.