Mr Edmonds, On Quasi-Radio-activity. 469 



Quasi- Radio-activity produced by the Point Discharge. By 

 S. A. Edmonds, A.K.C.Sc.L, " 1851 " Exhibition Science Research 

 Scholar, St John's College. 



[Received and read 16 May 1904.] 



Sella has shown* that when a metallic body is connected to 

 one terminal of an electrostatic machine, and made to face a series 

 of points connected to the other terminal of the machine, and the 

 discharge passed, without sparking, between them, for, say, an 

 hour, the body acquires an induced radio-activity if the discharge 

 has taken place in fresh air, and not the air of a room closed for 

 some time, while the presence near the body of thorium, during 

 the discharge, greatly enhances this radio-activity. The following 

 experiments were undertaken to discover the origin of this 

 activity. 



A series of discs were made of the metals aluminium, lead, 

 copper, zinc, iron, and brass, and also of cardboard lined with 

 tin foil and platinum foil respectively, their diameters being all 

 10 cms., and thicknesses 2 mm. ; a small hole, bored at the centre 

 of each one and recessed slightly, served to take a screw by means 

 of which they could be fixed, either to the terminal of the 6 plate 

 Wimshurst machine used in the experiments, or to the wire for 

 testing their activity. 



Another cardboard disc was prepared, lined with tin foil, and 

 12 needles thrust through it, their eye-ends being embedded in 

 a block of paraffin wax. This disc was mounted on a small 

 insulating upright, and placed so that the points of the needles 

 were some 10 cms. removed from the metal disc, at which distance 

 the discharge, in passing, just showed small brushes at the points, 

 the potential of which, as measured by an electrostatic voltmeter, 

 was 10,000 volts. 



The method used for testing the radio-activity is shown in the 

 diagram. 



A zinc cylinder C, G, of diameter 16 cms., was obtained, and 

 at its ends, one of which was removable, ebonite plugs D, D were 

 fitted, carrying brass rods, to one of which could be screwed the 

 metal disc to be tested, the other one carrying a smaller brass 

 disc with a guard-ring connected to the cylinder C which was 

 earth connected. 



The electrode B could be connected with the electrometer E, 

 an ordinary quadrant with single quartz fibre suspension and 



* Accad. Lincei Att. n. p. 57, Jan. 19; pp. 292—295, Mar. 16; pp. 369—373, 

 May 4, 1902. 



