Matter present in the Atmosphere. 105 



II. On the Collecting Distance of a Negatively-charged Wire. 



An estimate was next obtained of the approximate distance 

 from which excited activity can be drawn from the air to a 

 wire raised to a potential of —10,000 volts. On a still day 

 the amount collected on 6 metres of wire was almost the same 

 whether the wire was suspended at some distance from the 

 ground, or was placed within the zinc cylinder with open 

 ends. In the cylinder the potential gradient was steeper, 

 but the wire was to some extent screened from slight air- 

 currents. The collecting distance was therefore approxi- 

 mately equal to the radius of the cylinder, or to 40 cm. 



Two wires were then suspended in the air about 90 cms. 

 apart, and the third wire was fixed at a distance of 10 metres 

 from this. It was found that all three wires collected almost 

 the same amount of excited activity, so that the two neigh- 

 bouring wires were not drawing radioactive matter from 

 overlapping volumes. Similar experiments were made with 

 wires hung in the Engineering Building at various distances 

 from the outside of the tank. The experimental evidence is 

 not easy to summarize, but the general conclusion was obtained 

 that a wire raised to about —10,000 volts collected activity 

 from a cylindrical volume of about 10 to 80 cms. radius. This 

 result is not in agreement with previous suppositions, for 

 some observers have wrongly concluded that the collecting- 

 distance is very great. 



ft 



III. On the Rate of Production of Ions. 



It is important to obtain as many determinations as possible 

 of the value of q, or the rate of production of ions per 

 cubic centimetre of the air. A well-insulated wire was there- 

 fore hung down the middle of the large iron tank in the 

 Enoineerino- Building, and the wire was connected to the 



ft ft ft ' 



gold leaf system of the electroscope used throughout the 



experiments, but from which the zinc cylinder was now 



removed. On charging the wire to two or three hundred 

 o o 



volts, it was -found that a saturation current was not obtained. 

 Twelve wires were therefore taken (fig. 3), and fastened to a 

 lew zinc disks, 13 cms. in diameter, so that the wires were 

 along the generating lines of a cylinder of that diameter. 

 This system of wires, connected to the electroscope, and 

 charged to 300 volts, was found to give a saturation current. 

 By this arrangement, the whole tank practically became a 

 very large electroscope, and the discharge of the central wires 

 was due to the radioactive matter present in the air within 

 the tank. The capacity of the system was found by connecting 



