When a magnetic field is applied, each of the particles describes a curved path, whose radius of curvature depends on the velocity of the particle. For weak fields, only the particles of smallest velocity will be deflected sufficiently not to strike the outer cylinder, but, as the field is raised, the number will increase until finally all the β particles fail to reach the outer cylinder. The decrease of the charge communicated to the outer cylinder with the increase of the strength of the magnetic field is shown graphically in Fig. 30, Curve I. The ordinates represent in arbitrary units the charge communicated to the lead cylinder per second, and thus serve as a measure of the number of β particles which reach the cylinder. Knowing the dimensions of the apparatus, and assuming the value _e_/_m_ found by Kaufmann, the velocity of the particles which just fail to reach the lead cylinder can be deduced from any strength of the magnetic field. Curve II, Fig. 30 is the first differential of Curve I, and the ordinates represent the relative number of β particles which are projected at each velocity. [Illustration: Fig. 30.] From the data given by Kaufmann (see section 82) Paschen deduced that the group of rays examined by the former, which had velocities lying between 2·12 × 10¹⁰ and 2·90 × 10¹⁰ cms. per second, corresponded to the group of rays between the points _A_ and _B_, that is, to the group of rays which were completely deflected from the lead cylinder between the magnetic fields of strengths of 1875 and 4931 C.G.S. units. Since radium gives off β particles which require a field of strength over 7000 units to deflect them, Paschen concluded that β particles are expelled from radium with still greater velocities than the highest recorded by Kaufmann. Paschen considered that the small charge observed in still higher fields was mainly due to the γ rays. The effect is small and is probably not due to an actual charge carried by the γ rays but to a secondary effect produced by them. This question will be discussed in more detail in section 112. There is a group of low velocity β particles emitted by radium (see Fig. 30) which have about the same speed as the electrons set free in a vacuum tube. In consequence of their small velocity, these probably produce a large proportion of the ionization due to the β rays at short distances from the radium, for it will be shown (section 103) that the ionization produced by an electron per unit length of path steadily decreases with increase of its velocity above a small limiting value. This observation is confirmed by experiments on the absorption of the β rays in passing through matter. In Paschen’s experiments, the glass tube containing the radium was ·5 mms. thick, so that a considerable proportion of the low velocity β particles must have been stopped by it. This is borne out by some later experiments of Seitz which will be described in section 85.