J. Thomson gave great value to the
hypothesis by his measurements. At first he meant to determine the
speed of the cathode rays by direct experiment, and by observing, in a
revolving mirror, the relative displacement of two bands due to the
excitement of two fluorescent screens placed at different distances
from the cathode. But he soon perceived that the effect of the
fluorescence was not instantaneous, and that the lapse of time might
form a great source of error, and he then had recourse to indirect
methods. It is possible, by a simple calculation, to estimate the
deviations produced on the rays by a magnetic and an electric field
respectively as a function of the speed of propagation and of the
relation of the charge to the material mass of the electron. The
measurement of these deviations will then permit this speed and this
relation to be ascertained.
Other processes may be used which all give the same two quantities by
two suitably chosen measurements. Such are the radius of the curve
taken by the trajectory of the pencil in a perpendicular magnetic
field and the measure of the fall of potential under which the
discharge takes place, or the measure of the total quantity
of electricity carried in one second and the measure of the
calorific energy which may be given, during the same period, to a
thermo-electric junction.
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