120 BELL SYSTEM TECHNICAL JOURNAL 



recent achievement of Geiger and Klemperer. The process may be 

 called internal amplification of the primary ionization, the amplification 

 being in a constant proportion, or, as people carelessly call it, "linear." 



(C3) By developing an electrometer or electroscope so sensitive that 

 it is able to detect and even measure the total charge of a few thousands 

 of ions, without amplification. This was first achieved, or at any rate 

 applied to transmutation, by G. Hoffmann of Halle, and his associate 

 Pose; the latter was able to observe fragments of aluminium nuclei 

 (ejected by alpha-particles) which produced as few as three thousand 

 ion-pairs. The major difficulty seems to be, that the electroscope 

 takes a large fraction of a minute to perform its deflection and then 

 recover its readiness to respond to another particle. Pose in his 

 experiments observed only some thirty fragments to the hour.^^ 



(C4) By applying external amplification to the feeble impulse which 

 the primary ions due to a single particle produce in the external 

 circuit, and which is imperceptible to an electroscope of normal and 

 convenient quickness of response. This is done by developing the 

 superb techniques of amplification which modern vacuum-tubes have 

 rendered feasible, and like the three foregoing schemes is an achieve- 

 ment of the last few years, having been carried on especially by Wynn- 

 Williams of the Cavendish Laboratory and Dunning of Columbia. 



I show as Fig. 6 three records made with Dunning's apparatus, 

 wherein every vertical line is due to an ionizing particle, and is pro- 

 portional in length to the number of ions which the particle produced 

 in crossing a shallow chamber. ^^ The lines of great and nearly uniform 

 length which appear in record (a) are due to alpha-particles from 

 polonium; these all had nearly the same speed and were moving in 

 nearly parallel lines when they entered the chamber, and it is evident 

 that in crossing the gas they all made nearly the same amount of 

 ionization; they left with a good deal of their initial kinetic energy 

 unspent. The lines in record {b) are caused by protons; their diversity 

 in length is chiefly due to the wide variety of speeds which the protons 

 had when they entered the chamber, for these were fragments of the 

 disintegration of aluminium by alpha-particles, and therefore had a 

 broad distribution-in-speed (page 147). As these words imply, and 

 as I will stress presently, the ionization produced by a charged particle 



^^ It deserves to be recorded that in their blank experiments, Hoffmann and Pose 

 during one research observed deflections at the average rate of 1.22 per hour, but 

 observed altogether 197 of them ! 



" I am much indebted to Dr. Dunning for these pictures, made especially for this 

 article. He writes of {b): "The minimum amount of ionization detectable here is 

 well under 1000 ions; probably it could be pushed down to 250 ions." Consecutive 

 dots at the bottom of each record mark off the minutes. 



