562 Prof. L. Vegard on the Spectrum 



to the end of the glass tube, where the intensity is measured, 

 and the intensity I will be 



I = &2<r>%. 



When the field is put on, first of all the positive rays /^ 

 present at (A-B) are deflected into the glass wall, and then 

 the positive rays which on the way I are produced from the 

 neutral part are also brought out of the field of view of 

 the spectrograph. 



This gives for the observed moved intensity I m with a 

 magnetic field : 



Consequently : 



i_ 

 I m = h'W *i*e- H . 



I = fi 



T - 



L 2 is the mean distance which a neutral ray moves before it 

 takes up a positive charge, or "the mean free path "" of the 

 neutral ray. 



If we know L 2 , we shall be able to calculate the ratio 



I/Im. 



Now, according to Wien, "the mean free path " changes 

 comparatively little with the velocity of the rays, but very 

 considerably with the pressure in the observation chamber. 



Wien's measurements, however, are confined to fairly low 

 pressures, ranging from about 5 . 10~ 4 to 4 . 10 ~ 2 mm. Hg., 

 while the pressure in my experiments varied between 

 3'5 . 10" 2 and 10" 1 mm. Hg. 



Wien gives the following values : — 



For p — 0'0051 mm., the quantity L^ = 5*25 . 10" 5 cm. 



Po 



„ ^ = 0-039 „ „ „ L £ = 10 . 10- 5 cm. 



Po 

 p is atmospheric pressure, and L is defined by the 

 equation 



1 = i ^ 1 

 L hi L 2 



The ratio L 2 /^i is for the same pressures 6*1 and 2'G 

 respectively. L is "the mean free path " of the positivelv 

 charged caniers. 



" The mean free path " corresponding to the lowest 

 pressures used in my experiments can be found fairly 

 accurately from these values. In the case of the highest 

 pressure 0'1 mm., however, we have to extrapolate over a 

 fairly wide range ; so the value found for " the mean free 

 path' 5 will be somewhat uncertain. 



