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negatively charged and hurled into space with a great velocity. The existence of such a deflection has been demonstrated both by M. Dorn and M. Becquerel.

Let us consider the case of a ray which traverses the space situated between the two plates of a condenser. Suppose the direction of the ray parallel to the plates: when an electric field is established between the latter, the ray is subjected to the action of this uniform field along its whole path in the condenser $l$. By reason of this action the ray is deflected towards the positive plate and describes the arc of a parabola; on leaving the field, it continues its path in a straight line, following the tangent to the arc of the parabola at the point of exit. The ray can be received on a photographic plate perpendicular to its original direction. Observations are taken of the impression produced on the plate when the field is zero, and when it has a known value, and from that is deduced the value of the deflection, $\delta$, which is the distance of the points in which the new direction of the ray and its original direction meet a common plane perpendicular to the original direction. If $h$ is the distance of this plane from the condenser, i.e., at the edge of the field, we have, by a simple calculation,—

$\delta ={\frac {e\mathrm {F} l\left({\frac {l}{2}}+h\right)}{mv^{2}}};$

$m$ being the mass of the moving particles, $e$ its charge, $v$ its velocity, and $\mathrm {F}$ the strength of the field.

The experiments of M. Becquerel enable him to assign a value approaching to $\delta$.

Relation of the Charge to the Mass for a Particle Negatively Charged Emitted by Radium.

When a material particle having a mass $m$ and a negative charge $e$, is projected with a velocity $v$ into a uniform magnetic field perpendicular to its initial velocity, this particle describes, in a plane normal to the field and passing through its initial velocity, an arc of a circle of radius $\rho$, so that—$\mathrm {H}$ being the strength of the field—we have the relation—

$\mathrm {H} \rho ={\frac {m}{e}}v.$

If, for the same ray, the deflection, $\delta$, and the radius of curvature, $\rho$, be measured in a magnetic field, values could be found from these two experiments for the ratio ${\frac {e}{m}}$ and for the velocity, $v$.