706 Prof. E. Rutherford and Mr. S. J. Allen : Excited 



Elster and Geitel in their experiments have used an electro- 

 scope to measure the ionization produced. 



In order to produce a considerable quantity of activity on 

 the conductor it is necessary to charge the wire to a high 

 negative potential. Potentials varying from —5000 to 

 — 100,000 volts have been used in the experiments. 



A positively charged wire remains quite inactive however 

 long it may be exposed. 



Decay of Excited Radioactivity. 



The excited radiation from the air decays with the time in 

 a manner similar to the excited radiation from thorium and 

 radium, but at a different rate. The excited radiation from 

 thorium falls to half value in about 11 hours, while the 

 excited activity from air falls to half value in about 45 

 minutes for the range of voltages examined. It has been 

 shown* that the excited radiation from radium decays in an 

 irregular manner, the rate of decay depending on the time 

 of exposure. The rate of decay is rapid at first, then nearly 

 stationary for some time, and then a regular decay to zero, 

 falling to half value in about 30 minutes. It is thus seen 

 that the rate of decay of excited activity, due to the atmo- 

 sphere, is more nearly allied to that from radium than to that 

 from thorium. 



In the experiments detailed below the excited activity was 

 produced on a long insulated copper wire, 15 metres long, 

 suspended outside the laboratory window about 15 feet from 

 the ground. 



The wire was kept charged by means of a Wimshurst 

 machine driven by a motor. The potential of the wire was 

 measured by means of the sparking-distance between two 

 brass knobs. 



In order to regulate the potential of the exposed wire to 

 any desired value a needle-point connected to earth was 

 placed near a small plate connected to the charged wire. 

 The distance between the point and the plate was adjusted 

 until the spark just refused to pass between the knobs. This 

 method was found to be more satisfactory than varying the 

 speed of the machine. 



After the wire had been exposed a definite time, it was 

 rapidly removed and wound on a rectangular metal frame 

 120 cms. lono- and 10 cms. wide. 



* Rutherford. Phys. Zeit. xii. p. 254 (1002) ; aud Rutherford and 

 Miss Brooks, Phil. Mag-. July 1902. 



