PRODUCED IN GASES BY RONTGEN RAYS. 
201 
inner cylinder having the insulated juncture at k, the part D of the cylinder being 
connected to an electrometer. 
The gas stream is supposed to flow from right to left in the figure, and bdmn 
is the beam of rays. DB being at zero potential, suppose that when the potential 
of CC' is at a certain value the ions going towards DB move in paths parallel to the 
line ak in the upper half of the figure. An ion starting from any point to the 
left of ak would reach the part D and so influence the electrometer, but as all of the 
ions start from the beam of rays to the right of ak, all of them reach B. If the 
potential of CC' is diminished so that the inclination of the ionic paths becomes bk, 
ions from the outermost rim of bchnn will just begin to reach the part D. By a 
certain decrement in the potential of CC' the paths of the ions can be made 
parallel to dk, so that ions will reach D from a volume whose section is represented 
by the triangle bdg, the width of the beam of rays being bd. By a decrement in 
the potential of CC' equal to the last one, the volume from which ions reach D is 
increased by a volume whose section is seen from the figure to be nearly a parallelo¬ 
gram of about twice the area of the triangle bdg. Another equal decrement in 
the potential increases the volume by almost the same amount as the last. As 
the potential is diminished further, the rate of increase of volume gradually 
diminishes. So if we represent the potentials used by abscissas and the volumes 
from which ions reach D by corresponding ordinates, we obtain a curve, fig. 4, 
i ___L_l_ 
bed e f 
Fig. 4. 
whose inclination to the axis of abscissas, as the potentials are increased, at first 
gradually increases (RS of fig. 4), then assumes a constant value (ST) and finally 
diminishes (TU) as the curve ends in the axis of abscissas. The point U corre- 
YOL. CXCY.—A. 2 D 
