CONTEMPORARY ADVANCES IN PHYSICS 123 



produced by great numbers of electrons, and attempt to estimate these 

 numbers. This is done in the study of the beta-rays or fast electrons 

 emitted from radioactive nuclei, and in the study of cosmic rays; but 

 the method has not yet been applied to the rays emitted from atoms 

 undergoing transmutation by impacts, and apart from Joliot's observa- 

 tions on positive electrons (page 102 supra) nothing yet is known of 

 any electrons which may be emitted by these. 



Since individual electrons are so difficult or impossible to observe 

 by the customary methods, one might suppose that at any rate they 

 never annoy the observer. This unluckily is not so; for if electrons 

 are numerous, they may keep the electrometer needle (in the method 

 C4, for example) in a perpetual tremor, producing a so-called "back- 

 ground" over which even the strong sharp impulses due to alpha- 

 particles or protons may fail to stand out. It is even possible for a 

 chance coincidence or near-coincidence of several electrons to make a 

 record which cannot be disinguished from that of a single particle of 

 greater ionizing power. The scintillation-method suffers from a like 

 defect, for if the fluorescent screen is heavily bombarded with electrons 

 — or with gamma-rays, which liberate electrons from the fluorescent 

 stuff and the surrounding matter — it shines all over with a feeble 

 glow, against which the flashes made by more massive ions are difficult 

 to discern. The most casual student of transmutation cannot fail to 

 notice that polonium is generally used, of recent years, as the source 

 of alpha-particles for bombardment. Probably he infers that either 

 it is especially abundant or else supplies especially fast particles. 

 But in both respects polonium is inferior to another customary source, 

 radon mixed with its descendants radium A and radium B. It is 

 used because it emits no gamma-rays but feeble ones of low penetration, 

 whereas the other source pours out abundant and powerful photons 

 which flood any nearby ionization-chamber with electrons and confuse 

 the electrometer. Dunning's amplifying circuit, whereby he detected 

 charged nuclei set into motion by neutrons, was so devised as to 

 discriminate against the feeble but many impulses produced by these 

 electrons and in favor of the occasional stronger ones produced by the 

 massive particles; and this device enabled him to use, a source of the 

 latter type providing fifty times as many alpha-particles to engender 

 the neutrons, as the largest amount of polonium ever employed. 



Neutrons, I recall, are detected by observing the protons and more 

 massive nuclei which they convert by impact into fast-flying ionizing 

 particles, and photons by observing the electrons on which they have 

 the like effect; the problems of getting the data are thus not new, 

 it is the problem of interpreting them which is changed. 



