The photographic plate was at a distance h above the extremity of the field. Thus the particles struck the plate at a distance d_{2} from the original path given by
d_{2} = h tan θ + d_{1}
= (Xle/(mu^2))(l/2 + h).
In the experimental arrangement the values were
d_{2} = ·4 cms.;
X = 1·02 × 10^{12};
l = 3·45 cms.;
h = 1·2 cms.
If the radius R of curvature of the path of the same rays is observed in a magnetic field of strength H perpendicular to the rays,
e/m = V/(HR).
Combining these two equations we get
u = (X . l(l/2 + h))/(H . R . d_{2}).
A difficulty arose in identifying the part of the complex pencil of rays for which the electric and magnetic deviations were determined. Becquerel estimated that the value of HR for the rays deflected by the electric field was about 1600 C.G.S. units. Thus
u = 1·6 × 10^{10} cms. per second,
and e/m = 10^7.
Thus these rays had a velocity more than half the velocity of light, and an apparent mass about the same as the cathode ray particles, i.e. about 1/1000 of the mass of the hydrogen atom. The β ray is therefore analogous in all respects to the cathode ray, except that it differs in velocity. In a vacuum tube the cathode rays generally have a velocity of about 2 × 10^9 cms. per sec. In special tubes with strong fields the velocity may be increased to about 10^{10} cms. per sec. These β particles, then, behave like isolated units of negative electricity, identical with the electrons set free by an electric discharge in a vacuum tube. The electrons projected
