910 Prof. H. A. Bumstead on Emission of Electrons 



a clear opening of 3*2 cm. The brass rod which supports 

 the ring passes through the base-plate C, and is insulated 

 from it by amber, ebonite, and an earthed guard-tube ; the 

 joints are made air-tight with sealing-wax. A cylindrical 

 cover B rests on the base-plate, the two surfaces in contact 

 being ground to each other and made tight with rubber stop- 

 cock grease. The interior is connected by means of the tube 

 L with pump, gauge, and charcoal-bulb. The copper plug 

 P, which has the polonium on its lower end, is supported by 

 a little tripod, and there are punch marks in the top of the 

 cover into which the feet of the tripod fit, so that it can be 

 removed and replaced in the same position. The distance 

 from the polonium to the top of the cover is 6 mm. In the 

 cover, just below the polonium, are nine holes, 1 mm. in 

 diameter, to permit the passage of the a-rays ; these holes are 

 covered by aluminium foil, 8*65 x 10 -4 cm. thick. This foil 

 was some that was supplied with tubes intended to demon- 

 strate Lenard rays ; it was the thinnest I could find which 

 was not full of holes, and its effect on the range of the 

 a-particles was equivalent to that of 1*47 cm. of air. It was 

 fastened down to the top with a ring of low melting sealing- 

 wax, which was then covered with stopcock grease. 



The rod which supports the ring and aluminium foil 

 was connected to the gold leaf of an electroscope which had 

 been made some time before in this laboratory. It may be 

 regarded as developed from a Wilson tilted electroscope in 

 the same manner that a twinned crystal is developed from an 

 ordinary one. The two plates H and IP are charged to 

 equal and opposite potentials (usually 200 volts) as in Hankel's 

 electroscope. The gold leaf is- kept in the middle by means 

 of the levelling- screws, and the sensitiveness (and stability) 

 are easily altered by raising or lowering the leaf by means of 

 the milled head M. The latter adjustment is the chief 

 convenience of this electroscope. The sensitiveness of such 

 an electroscope is a very indefinite thing ; it depends upon 

 the degree of instability of zero point and deflexion that one 

 can permit, and this, in turn, depends on the greater or less 

 protection of the case from rapid changes of temperature, 

 currents of air, &c. This double electroscope appears to 

 have some slight advantage over Wilson's form ; when the 

 two were mounted side by side under the same conditions 

 the double electroscope had a sensitiveness about three times 

 that of the tilted form, for equal stability. As used in the 

 present experiments, the capacity of the leaf and its con- 

 nexions was about 5 cm. and the sensitiveness was so adjusted 

 as to oive about 25 or 30 divisions on the scale in the 



