172 SUMMARY OF ELECTRON THEORY [CH. XVIIK 



slightly, in a direction indicating that they are posi- 

 tively charged particles. Rutherford and Becquerel 

 have both observed this fact, and Strutt has confirmed 

 the fact of positive charge. But quite recently* 

 Soddy has surmised that this positive charge might 

 be acquired by ionisation in travelling through the 

 air, and that in a high enough vacuum no deflexion 

 would be observed ; thereby showing that intrinsi- 

 cally they did not possess a charge. He believes 

 himself to have confirmed this by careful though 

 difficult experiment. So important, and in some 

 respects improbable, a conclusion, however, cannot 

 yet be regarded as at all certain. 



Rutherford was the first to observe the fact and 

 the sign of the magnetic deflexion or curvature of 

 alpha rays, and to make an estimate of its amount. 

 He invented the device of sending them through a 

 magnetic field, up a stream of rarified hydrogen, 

 between a set of narrow plates set edgeways ; which 

 latter constituted a grid that would be opaque 

 unless the trajectories of the flying particles were 

 rigorously straight. He thus made the discovery that 

 the rays consisted of positively charged particles, and 

 arrived at a rough estimate of their atomic weight. 



Becquerel measured the magnetic deflexion of 

 alpha rays, at different distances from the source, 

 by letting them graze a photographic plate at a 

 known angle. The actual trace observed was a 

 short slant line with a slight curvature ; reversal of 

 the field slanted the line the other way, thus giving 

 a resultant impression like the conventional two wings 

 of 'a flying bird drawn at a very acute angle. Subse- 

 quent measurement of the distance apart of different 

 positions of the two wings gave the data sought. 



* See Nature, 2nd August, 1906. 



