118 BELL SYSTEM TECHNICAL JOURNAL 



Laboratory and the Institut fur Radiumforschung in Vienna have 

 ever submitted themselves to the long, tedious and nerve-racking 

 process of counting thousands of dim flashes for periods of hours in 

 darkened rooms with dark-adapted eyes; and if two disagreed as to 

 what was observed, there was no objective way of deciding between 

 them. The newer methods abolish this strain; they can readily be so 

 shaped as to leave a permanent record, which anyone may consult and 

 analyze for himself; and they are capable of measuring the ionizing 

 power of the fragments. Nevertheless the eldest method still retains 

 the unique advantage that no barrier whatever, not even a gas, need 

 intervene between the detecting screen and the source of the fragments; 

 and also it is often employed by those accustomed to scintillations as 

 a check upon the others. 



The second method (B) is that of the expansion-chamber or cloud- 

 chamber of C. T. R. Wilson, whereby the tracks of ionizing particles 

 across a gas are made visible by droplets of water which condense 

 upon the ions. This is the splendid invention which is the joy of all 

 who write or lecture on atomic physics, since it enables them to deco- 

 rate their exposition with pictures which make real the things of 

 which they speak. It has virtue for the investigator also, especially 

 since it may show in a single vivid photograph how many fragments 

 there are formed in a single process, what are the directions in which 

 they fly away, and how far they are able to travel through the gas. 

 The curvature of the track in an applied magnetic field supplies the 

 value of the momentum of the particle which made the track, if the 

 nature of the particle be known; and this last may often be guessed 

 from the aspect of the track, or assured by independent data. The 

 major disadvantage of the method is, that the apparatus records only 

 the particles which fly off during about a hundredth of a second, and 

 then lies idle for several seconds or even minutes while it is being 

 prepared for its next brief interval of effectiveness. 



The third method (C) — or group of methods rather, for the variants 

 are legion — is the detection by purely electrical methods of the ions 

 which the fragments produce as they shoot across the gas of an 

 ionization-chamber. A fast-flying charged particle loses on the aver- 

 age 30 to 35 electron-volts for every ion, or rather every ion-pair, 

 which it produces.'^ To see the utility of this theorem, turn it around ; 

 the number of ion-pairs produced by a fast charged particle going 

 through a gas is about a thirtieth of the number of electron volts 

 which it loses in its transit. A fast alpha-particle, such as are spon- 

 taneously emitted by radon, or constitute the fragments springing out 



'' "Electrical Phenomena in Gases," pp. 52, 70-71. 



