406 H. A. Bumstead — Emission of Electrons hy 



In order to avoid possible complications from the effects of 

 /3- and 7-rays, polonium was used as the source of a-rays. Pro- 

 fessor Boltwood kindly separated the polonium from a solution 

 of radio-lead and it was deposited on the end of a copper plug, 

 4""" in diameter. The first preparation was not very strong 

 and it was necessary to use a system of small capacity in 

 making the electrical measurements. The following form of 

 apparatus was used. A piece of thin aluminium leaf, E, tig. 1, 

 •064 XlO -1 ™ 1 thick is stretched over the flat brass ring, D, which 

 has a clear opening of 3"2 em . 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, l mm in diameter, to permit the passage of the a- 

 rays; these holes are covered by aluminium foil, 8 - 65xl0~ 4cm 

 thick. This foil was some that was supplied with tubes 

 intended to demonstrate Lenard rays ; it was the thinnest I 

 could find which was not full of holes and its effect on the 

 range of a-particles was equivalent to that of l-4T cm of air. 

 It was fastened down to the top with a ring of low melting 

 sealing wax, which was then covered with stop-cock grease. 



The rod which supports the ring and aluminium foil was con- 

 nected 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 JB7, are charged to equal and opposite poten- 

 tials (usually 200 volts), as in TIankel's electroscope. The gold 

 leaf is kept in the middle by means of the leveling 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 

 deflection 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, etc. This double electroscope ap- 

 pears to have some slight advantage over Wilson's form ; when 



