[m'lennan] on electric CHARGES 87 



pressure. When the pressure was taken below 1/100 of a millimetre the 

 charging action became very marked and produced rapid deflections 

 beyond the limits of the scale. 



A set of readings taken with tliis polonium coated copper strip in 

 the neighbourhood of the initial rise due to the radiation is given in 

 Fig. 1, and a curve drawn from the readings is shown in the same figure. 

 This curve is typical of the different ones obtained with this active pro- 

 duct. 



îH 



r; k,-l-r ' l .'I; I .1 , l-:.l: 'a'. -I.^M i..Hih|ii!.l):,'l liHi;!f;:iiiHiHiift[tt}} 



.1 ^ 



Fig. I. Fig. IL 



It shows quite clearly that this substance emits an excess of nega- 

 tively charged particles, and it also exhibits very definitely the pressure 

 at which this excess comes into evidence. 



III. Charging Action of Secondary Eays Excited in Aluminium 

 BY THE Gamma Eays From Radium. 



In this experiment one milligram of radium was enclosed in a small 

 sealed glass tube, whose walls were about 2 mms. thick. This glass tube 

 was placed inside a tube of aluminium, which was then closed at the top 

 and bottom. This tube, which was about 7 cm. in length and had walla 

 about 2 mms. in thickness, was then insulated as before and suspended in 

 the brass cylinder used in the last experiment. In this experiment the 

 charging action was very much more marked than in the experiments 

 with polonium. It also exhibited certain characteristics which were not 

 observed in the measurements with that substance. 



A set of results which illustrate its behaviour is given in Table II, 

 and a curve plotted from them, is shown in Fig. II. When the outside 

 vessel and the free system were joined to earth the zero reading on the 

 scale was 20, and as soon as the earth connection to the quadrants was re- 

 moved the needle moved out at once in the negative direction and took 

 up a position at 13.3. When the pressure was lowered the negative 

 charge on the quadrants gradually increased, but ultimately at a pres- 

 sure of about 80 mm. this acquisition of a negative charge ceased and 



