the magnetic permeability of the medium. We then have the easily established relation
The β-rays are also deviated in an electric field. Suppose we have a rectilinear pencil of these rays. If a uniform electric field be produced normal to the initial direction of the pencil, the rays are deviated in the opposite direction to the field, and describe parabolic paths. The experiment can be performed by passing the pencil of rays between two parallel metallic plates, between which there is established a difference of potential. The deviation is slight with the means one has at command, and it is convenient to work in a vacuum. The air is made a conductor of electricity, so that if we work in the air the insulation is imperfect, and it is difficult to maintain a constant and high difference of potential between the plates. The most penetrating β-rays are bent aside least.
The action of the electric field is in accord with the ballistic hypothesis previously stated. Accepting this hypothesis, suppose a uniform electric field with an intensity
and a width
, acts upon the charged projectile whose initial velocity is normal to the field. The deviation,
, of the extremity of the trajectory at the departure from the field is given by formula (2), when the deviation is slight.