CONTEMPORARY ADVANCES IN PHYSICS 



349 



of Lawrence and Edlefsen's figure (relative ordinates have no signifi- 

 cance, it is only the trends of the curves which should be compared). 

 Mohler and Boeckner also measured the actual number of ions 

 produced by light of known intensity in a known quantity of gas, 

 using of course the absolute method, and expressing their results in the 

 following way. Suppose a thin stratum of gas, of thickness dx and 

 area A. Denote by N the number of atoms per unit volume of the 



2400 2600 2800 3000 3200 



ANGSTROMS 



3400 



3600 



3800 



Fig. 2 — Ionization by light plotted as function of wavelength for rubidium (Criti- 

 cal wavelength at 2968A). Circles and crosses correspond to different densities. 

 (E. O. Lawrence, N. Edlefsen.) 



gas; then NAdx will stand for the number in the stratum. Denote 

 by Q the total number of photons striking the stratum in unit time; 

 suppose that they fall upon it perpendicularly, and are evenly dis- 

 tributed over its area. The number of ions formed in the stratum 

 in unit time, / will be proportional to NAdx and to Q/A. Write: 



/ = kNQdx 



the coefficient k is the quantity of which the experiments are designed 

 to reveal the value. (We should not be entitled to expect this to be 

 constant, if more than a small fraction of the quanta were spent in 

 ionization; but in the practical cases we may.) The values which 

 they give are (2.3 ± 0.2) • 10~^^ for caesium and 1.1 • 10~^^ for rubidium, 



