the velocity of the particle, but can only alter the direction of its path.
If ρ is the radius of curvature of the path of the ion, the force along the normal is equal to
and this is balanced by the force Heu sin θ.
If
i.e. if the ion is moving at right angles to the direction of the magnetic field
or
Since u is constant, ρ is also constant, i.e. the particle describes a circular orbit of radius ρ. The radius of the circular orbit is thus directly proportional to u, and inversely proportional to H.
If the ion is moving at an angle θ with the direction of the magnetic field, it describes a curve which is compounded of a motion of a particle of velocity u sin θ perpendicular to the field and u cos θ in the direction of the field. The former describes a circular orbit of radius ρ, given by
the latter is unaffected by the magnetic field and moves uniformly in the direction of the magnetic field with a velocity u cos θ. The motion of the particle is in consequence a helix, traced on a cylinder of radius
whose axis is in the direction of the magnetic field. Thus an ion projected obliquely to the direction of a uniform magnetic field always moves in a helix whose axis is parallel to the lines of magnetic force45.
50. Determination of e/m for the cathode stream. The cathode rays, first observed by Varley, were investigated in detail by Crookes. These rays are projected from the cathode in a vacuum tube at low pressure. They travel in straight lines, and are readily deflected by a magnet, and produce strong luminosity in a variety of substances placed in their path. The rays are deflected by a magnetic field in the same direction as would be expected for a negatively charged particle projected from the cathode. In order to explain the peculiar properties of these rays Crookes supposed that they consisted of negatively electrified particles, moving with great velocity and constituting, as he appropriately termed it, “a new or fourth state of matter.” The nature of these rays was for twenty years a subject of much controversy, for while some upheld their material character, others considered that they were a special form of wave motion in the ether.
Perrin and J.