140 Mr. S. J. Allan on the 
would be shown. Such, however, has not proved to be the 
ease. Wires 2050 and 4090 cms. long were carefully 
explored, the action on the wire being produced by means 
of an oscillator having plates 40 cms. square and the straight 
connexion between 60 cms. long, but the only unmistakable 
minimum was approximately 150 cms. from the free end, 
the same as was perfectly formed with wires from 300 cms. 
upwards. 
Some evidence was obtained as to the cause of the marked 
minimum near the other end of the wire. It was due to the 
direct action of the oscillator on the detector. As described 
in the other paper, an attempt had been made to allow for 
this direct action by taking the reading when the wire was 
in place and also when it was removed, ; and then subtracting 
the latter from the former. This assumes that the two 
effects are quite independent, but such seems hardly to be 
the case. In the former experiments the detector lay in a 
horizontal plane on the top of a carriage which was moved 
along the wire. Thus the detector’s length was parallel to 
that of the oscillator, though the little wing was perpendicular 
to it. As described in Section II. of the present paper, the 
detector was now hung ina vertical plane from the wire, 
so that its length was perpendicular to the axis of the 
oscillator. With this arrangement the minimum disappeared, 
thus showing that it had been produced by the direct action 
of the oscillator on the detector. 
University of Toronto, 
Toronto, Canada. 


XIV. Foe of the eee By 8. J. ALLAN, 
ML.Se., Demonstrator in Physics, Me Gill University * 
[Plate XTV.] 
ie a previous papery some experiments were described 
dealing with the rate of decay and penetrating power of 
the excited radioactivity obtained from the atmosphere on a 
negatively-charged wire. It was found that the activity 
decayed according to an exponential law with the time, 
falling to half its value in 45 minutes. Its penetrating 
power was slightly greater than that of the excited activity 
from radium or thorium. Its absorption by solids followed 
an exponential law with the thickness, and the radiation was 
cut down to half its value by -001 cm. of aluminium. 
The amount of excited activity that could be obtained from 
* Communicated by Prof. FE. Rutherford, F.R.S. 
+ Rutherford and Allan, Phil. Mag. Dec. 1902. 

