420 Prof. McLennan on Induced Radioactivity 



peculiar constituent similar to the emanation from thorium, 

 which has been shown by Rutherford * to induce radioactivity 

 in any body with which it comes in contact, especially when 

 that body is negatively electrified. 



The difficulty of determining and of regulating the atmo- 

 spheric conditions for observations upon this excited or 

 induced radioactivity suggested the desirability of resorting, 

 for purposes of experiment, to a locality where exceptional 

 electrical conditions were known to exist permanently in the 

 atmosphere. Niagara Falls, according to Lenard's results, is 

 preeminently such a locality, and, through the kindness of 

 the Hon. Thomas Walsh, Superintendent of the Niagara Falls 

 National Park Reservation, the author was enabled in Sep- 

 tember last to carry out a short series of observations upon 

 excited radioactivity at the foot of the Falls. The general 

 result of the investigation was that during the course of the 

 experiments the amount of radioactivity induced in a wire 

 exposed at the foot of the Falls was found to be very much 

 less than that in a wire exposed in the same manner in 

 Toronto. 



2 . — Apparatus* 



In these observations the measurements were made with a 

 quadrant electrometer of the Mascart type as constructed by 

 Oarpentier. The silk suspension in the original apparatus 

 was replaced by a phosphor-bronze strip less than 0*025 milli- 

 metre in thickness, which was attached at its upper end to 

 an ebonite rod to secure insulation. The needle was kept 

 charged by a battery of small storage-cells similar to those 

 installed in the Reichsanstalt. The deflexions were measured 

 by the movement of the image of an incandescent-lamp 

 filament upon a transparent scale placed at a distance of one 

 metre from the electrometer. 



With a potential of 480 volts applied to the needle, the 

 sensibility of the instrument was such as to produce a 

 deflexion of 1000 millims. on the scale for a potential- 

 difference of one volt between the quadrants. In measuring 

 the induced radioactivity, bare copper-wire No. 24 was ex- 

 posed in the open air by means of specially-constructed 

 insulating supports (fig. 1) attached to a series of bamboo 

 poles erected at convenient distances. The wire was charged 

 oy a small Toepler Holtz Electrical Machine driven by a 

 water motor, which maintained a potential of from eight to 

 ten thousand volts. 



The insulator shown in fig. 1 consisted of a brass .tube 

 * Rutherford, Phil. Mag-, xlix. p. 1 & p. 161 (1900). 



