On the Ionisation of Atmospheric Air. 



155 



could from time to time be made up by contact with a rubbed ebonite 

 rod or a miniature electrophorus. 



Both the gold leaf of which the motion served to measure the 

 leakage which was the subject of investigation, and that of the Exner 

 electrometer, were read by means of microscopes provided with eye- 

 piece micrometers. 



To give the leaking system an initial potential equal to that of the 

 supporting rod, momentary electrical connection between them was 

 made by means of a magnetic contact-maker. This consisted of a fine 

 steel wire fixed to the supporting rod near its upper end and extend- 

 ing just below the sulphur bead, where it was bent into a loop 



Fig. 1. 



surrounding the prolongation of the brass strip which carried the gold 

 leaf. A magnet brought near the outside of the tube attracted the 

 wire till the loop came in contact with the brass and brought it into 

 electrical communication with the supporting rod. This operation 

 was repeated every time the potential of the leaking system had fallen 

 so far that the gold leaf approached the lower end of the scale. The 

 potential of the supporting rod was not allowed to vary by more than 

 a very few volts, and before each reading of the potential of the leak- 

 ing system was always brought to within a fraction of a volt of its 

 initial value ; the Exner electroscope served to indicate when this was 

 the case. The initial difference of potential used in most of the 

 experiments amounted to about 200 volts. 



To determine the fall in potential corresponding to a movement of 

 VOL. LXVIII. N 



