SOME CONTEMPORARY AlWANCES IN PHYStCS-Vl 127 



FiK- 5 — Logarithmic plot of the currents across a gas (air) in which ionization by 



collision is occurring, for a constant fieltl strength and various thicknesses of gas 



(Data from Townscnd) 



Experimentally, the test of (5) is made by dividing each one of 

 Townsend's values of a by the pressure at which it was determined, 

 and then plotting all these values of a/ p versus the corresponding 

 values of X p. All the points for any one gas should lie on or close 

 to a single curve, and within certain ranges of pressure and field 

 strength they do; so far, good. The curve should be an exponential 



