CONTEMPORARY ADVANCES IN PHYSICS 



597 



later in another meaning), and a for the ratio «i/w, we obtain: 



k' = on J 2.2 2^ {V[l+w'0V(l - a')J -i- n-'eh'a' 

 2(1 + n^d^T^) 



+ [I +n20V(l - a^)]}. (17) 



By measuring k with waves of a single frequency, and measuring in 

 addition either of the two quantities N and g, it is possible to evaluate 

 the other of the two by means of this equation ; provided the equation 

 is correct, a presumption which should be tested by making the 

 measurements at several different frequencies. 



Fig. 7 — ^Apparatus for measuring the absorption of Hertzian waves by ionized 

 gas. S, source of waves; G, semitransparent mirror; Ei, £2, receivers; A, F, anode 

 and cathode of ionizing discharge; L, wire probe. (W. Hasselbeck; Annalen der 

 Physik.) 



Danzer sent 4-cm. waves (of frequency 7.5- 10^)through a stratum 

 of neon-helium mixture excited by a low-frequency discharge; measured 

 the absorption, and measured also g by the method later employed 

 by Appleton and Chapman. For N he then computed the value 

 1.3- 10^2 (electrons per cc.) — a value of which, in the lack of further 

 knowledge concerning electrons in this gas (he does not even state 

 the pressure) one can only say that it is probably of the right order 

 of magnitude. Much more extensive was the work of Hasselbeck, 

 who used various frequencies ranging from 4.8 to 1.44 times 10^ 

 and introduced a probe into the gas in order to measure N and the 



