RADIO-ACTIVITY 193 



directly or Indirectly, gives rise to some 40,000 

 ions or more, enough to give the gas a measurable 

 conductivity. If, therefore, an electrometer be 

 inserted in the circuit, the flight of each a particle 

 is shown by a transient deflection — by a visible 

 movement of a spot of light reflected from the 

 needle of the instrument on to a scale. It will be 

 noted that, in these two ways, Rutherford and 

 Geiger detected the effect of an individual atom — 

 a crowning verification for Dalton's atomic theory. 



The number of a particles emitted persecond by 

 the product radium C in radio-active equilibrium 

 with I gram of radium was thus estimated at 

 3.4 X lo^^ The same number must be emitted 

 by the gram of radium itself, and by each of its 

 three a ray products — radium emanation, radium 

 A, and radium C. Consequently i gram of radium 

 and its products in equilibrium emit 13.6 x 10^^ a 

 particles per second. 



Having counted the number, the next thing 

 was to estimate the total charge of the particles. 

 The a rays are intercepted by an insulated plate, 

 and the gain in electric charge by the plate is 

 measured by an electrometer. The charge carried 

 by each a particle was found to be 9.3 x io~^^ 

 electrostatic units. This result was confirmed by 

 Regener, who obtained the figure 9.6 x io~^*^ from 

 radium F. 



Now Mllllkan's value for the single unit 

 charge carried by electrons on univalent ions is 

 4.77 X io~"^° electrostatic units. Thus it is clear 

 that the a particle carries two positive electric 

 units, and has a mass four times that of a hydrogen 

 atom — that it is, in fact, a helium atom with two 

 unit charges, shot forth with high velocity. 



o 



