It was shown by Geitel,* and independently by the writer of this paper,! that there is a continuous production of ions in air contained in a closed vessel, even when it is not exposed to any known ionising agent. The present communication contains the results of measure- ments of the relative rates of production of ions under such conditions in air and certain other gases.
The relative ionisations (the term " ionisation " being used in the sense of " rate of production of ions ") were obtained by a comparison of the " saturation " currents in the various gases. The apparatus used for this purpose differed only in minor details from that described and figured in a previous paper 4
The vessel in which the ionisation was measured was a glass bulb 5 '5 cm. in diameter, thinly silvered internally. The conducting system, from which took place the loss of charge to be measured, consisted of a brass wire with a narrow strip of gold leaf attached, the wire being fixed by means of a small bead of sulphur to a brass rod passing through a plug of sealing-wax in the neck of the bulb. This supporting rod was kept at a constant potential by means of a condenser, con- sisting of zinc plates embedded in sulphur. Electrical connection could be made when required between the leaking system and the supporting rod by means of a strip of watch balance-spring, soldered at its upper end to the supporting rod, and with its lower end bent so as to form a bridge over the sulphur bead. By approaching a magnet to the neck of the bulb the free end of the spring could be brought into contact with the upper end of the brass wire carrying the gold leaf.
The position of the gold leaf was read by means of a microscope provided with an eye-piece micrometer-scale. On the portion of the scale used, one scale division corresponded to 0*31 volt, as tested by means of a Clark cell. An initial potential of about 90 volts was used in the experiments.
To make a determination of the rate of leak, the brass wire and gold leaf were brought to the potential of the supporting rod by making momentary electrical connection by means of the magnetic 'contact maker. The time indicated by a watch at the moment of transit of the gold leaf across the next scale division was then observed ; the moment of transit across a lower scale division about one hour later was also observed. Contact was then made a second time and the position of the gold leaf again read. The time taken by the gold leaf to traverse a certain whole number of scale divisions was thus deter-
- ' Physik. Zeitschrift,' 2 Jahrgang, No. 8, pp. 116-119.
t ' Camb. Phil. Soc. Proc.,' vol. 11, p. 32 ; ' Roy. Soc. Proc.,' vol. 68, pp. 151-161.
t ' Koy. Soc. Proc.,' vol. 68, p. 155.
