Excited Radioactivity and Ionization of the Atmosphere. 705 



sensitive quadrant electrometer for examination both of the 

 ionization and excited radioactivity produced in air. 



The electrometer employed is a modification o£ the Dole- 

 zalek electrometer which is described in Tnstrumente ICunde, 

 Dec. 1901. It is o£ the ordinary quadrant type, with a very 

 light needle of silver paper suspended by a fine quartz 

 fibre. The apparatus, as constructed by Herr Bartels, of 

 •Gottingen, was for determination of small P.Ds. for electro- 

 chemical work. For our purpose it was necessary to com- 

 pletely alter the insulation and method of connexion of the 

 quadrants. In the present experiments the needle was 

 charged at intervals of two days by lightly touching the 

 needle by a fine wire connected to a battery of 200 volts. 

 It was found that the needle did not lose more than 10 per 

 •cent, of its charge in 21 hours. The damping of the needle, 

 on account of its lightness, was fairly rapid, and no extra 

 damping vane was required. The deflexion was observed by 

 a telescope and scale at a distance of 2 metres. The zero 

 point was found to be very steady. For the first suspension 

 employed the electrometer gave a deflexion of about 1800 rams. 

 of scale corresponding to one volt P. D. between the quadrants, 

 when the needle was charged to 200 volts. This suspension 

 was accidentally broken in the course of the experiments and 

 was replaced by a quartz fibre which gave only about 5- of 

 this deflexion for the same voltage. When dealing with 

 the very small rate of discharge which is produced by the 

 spontaneous ionization of air, it is very essential that every 

 precaution should be taken to guard against external electro- 

 static disturbances. The electrometer and all the connecting 

 wires were inclosed in gauze cylinders connected to earth. 

 The floor and woodwork in the immediate neighbourhood of 

 the testing apparatus were covered with metal connected to 

 earth. The separation of the quadrants was done by means 

 of a special mercury key operated from a distance by a 

 cord. 



The insulating substances necessary in experimental 

 arrangements were completely diselectritied by means of 

 flames. 



Production of Excited Radioactivity. 



The simplest method of obtaining a large amount of excited 

 radioactivity from the air is to expose a long insulated wire 

 charged to a high negative potential in the open air. After 

 exposure for several hours the wire is removed and wound 

 on a frame, or in the form of a flat helix. The ionization 

 produced by the radioactive wire in the testing vessel is then 

 observed,! by means of the electrometer, in the usual way. 



Phil. Mag. S. 6. Vol. 4. No. 21. Dec. 1902. 3 A 



