158 S. G. LUSBY AND T. EWING. 
THE RATE or DECAY oF THE EXCITED RADIO- 
ACTIVITY FROM THE ATMOSPHERE 1n SYDNEY. 
By 8S. G. LusBy, B.A. and T. EWING, B.Sc. 
(Communicated by Prof. POLLOCK.) 
[Read before the Royal Society of N. S. Wales, December 5, 1906. ] 
IN connection with an investigation, still in progress, it 
was considered necessary to find the rate of decay of the 
excited radioactivity from the atmosphere in Sydney. 
Although the work consisted merely of a repetition of an 
experiment of Allan,’ following work of Rutherford,’ Elster 
and Geitel,’ and Rutherford and Allan,* the result may be 
of some interest as no determination of the constant made 
in Australia has, so far as we know, been published. The 
facts are here separately recorded as the main research is 
no longer directly concerned with the matter. 
A copper wire, ten metres in length was suspended 
vertically in the tower of the Physical Laboratory of the 
University; it was kept negatively charged by a Wimshurst 
machine, the potential being about 25,000 volts. After 
about three hours exposure, the wire was removed and 
rubbed with cotton wool moistened with ammonia. The 
cotton wool was then incinerated in a platinum crucible 
and the ashes tested for radioactivity in the usual way. A 
Dolezalek electrometer was used and the whole of the 
apparatus screened from outside electrostatic influence. 
The observations were commenced about 15 minutes after 
the wire was discharged, the activities being measured in 
terms of scale divisions per second. 
1 Phil. Mag., Feb. 1904. % Phil. Mag., Feb. 1900, * Phys. Zeit., iii. 
and x],,1901. * Phil. Mag., Dec. 1902. 
