Ill' II A. Bum stead — Emission of Electrons by 



decrease, instead of increasing, the effect. Unfortunately when 

 this was tried a decisive result was not obtained. The cur- 

 rent observed in the electroscope remained nearly constant 

 while several foils were interposed and then fell off more 

 gradually than with the positive charge on the case. As will 

 be shown in the following pages, however, this was not due to 

 the cause suggested above, but was in all probability the result 

 of the construction of the chamber. As has been pointed out, 

 many of the a-rays must have struck the sides of the holes in 

 the top at various angles up to grazing incidence. Thus the 

 electrons emitted from the case were not all produced by par- 

 ticles moving with approximately the same speed, but by par- 

 ticles whose velocities varied considerably. Hence the effect 

 was somewhat analogous to ionization curves obtained when 

 the pencil of rays is not limited to a small angle by a " Bragg 

 screen." 



However, it was necessary to investigate the matter further. 

 As a more intense source of radiation was desirable, Professor 

 Boltwood was good enough to attempt the preparation of a 

 much stronger deposit of polonium. By a special method he 

 succeeded in depositing upon a 4"' m plug as much polonium as 

 would be in equilibrium with about - 6 mg of radium, and yet 

 having so little foreign material with it that it appeared as a 

 mere discoloration upon the copper. With this preparation, 

 the difficulties of the experiment were greatly lessened ; an 

 electrometer could be used instead of the electroscope, and much 

 larger and steadier readings could be obtained. 



In order to avoid the difficulties mentioned above, which 

 prevented satisfactory readings wdien a negative potential was 

 put on the case, the experimental arrangements were altered. 



The polonium-coated plug, F (fig. 3), is placed within the 

 evacuated chamber ; it is surrounded by a brass cylinder, C, 

 whose opening limits the cone of rays so that they fall within 

 the ring, E, wdiich supports the aluminium-leaf electrode. Two 

 other rings, F and F', above and below the electrode at a dis- 

 tance of 7 mm , are also covered with the aluminium leaf; they 

 are in metallic connection with each other and with the case. 

 The brass disc, D, is divided into six equal sectors ; one of 

 these is left blank so that the rays may be stopped, while the 

 five others have holes through them l-3 cm in diameter. These 

 five holes are covered respectively with 2, 3, 4, 5, and 6 layers 

 of the thicker aluminium foil whose air equivalent is - 58 cm . 

 The disc can be rotated about its axis, which passes through 

 the cover plate with a cone bearing, made tight by rubber 

 stop-cock grease. A dial, D', enables one to set the disc so 

 that the different layers of foil are interposed, or the rays 

 stopped altogether by the blank sector. The electrode is insu- 



