106 NATURE OF THE RADIATIONS [CH. 
Dorn! and Becquerel? showed that they also were deflected by an 
electrostatic field. 
By observing the amount of the electrostatic and magnetic 
deviation, Becquerel was able to determine the ratio of e/m and 
the velocity of the projected particles. Two rectangular copper 
plates, 3°45 cms. high and 1 cm. apart, were placed in a vertical 
plane and insulated on paraffin blocks. One plate was charged to 
a high potential by means of an influence machine, and the other 
was connected to earth. The active matter was placed in a narrow 
groove cut in a lead plate parallel to the copper plates and placed 
midway between them. The photographic plate, enveloped in 
black paper, was placed horizontally above the plate containing 
the active substance. The large and diffuse pencil of rays thus 
obtained was deflected by the electric field, but the deviation 
amounted to only a few millimetres and was difficult to measure. 
The method finally adopted was to place vertically above the 
active matter a thin screen of mica, which cut the field into two 
equal parts. Thus, in the absence of an electric field, a narrow 
rectangular shadow was produced on the plate. 
When the electric field was applied, the rays were deflected and 
a part of the pencil of rays was stopped by the mica screen. A 
shadow was thus cast on the plate which showed the direction of 
deviation and corresponded to the least deviable rays which gave 
an impression through the black paper. 
If a particle of mass m, charge e, and velocity v, is projected 
normally to an electric field of strength X, the acceleration a is in 
the direction of the field, and is given by 
m- 
Since the particle moves with a constant acceleration parallel to 
the field, the path of the particle is the same as that of a body 
projected horizontally from a height with a constant velocity and 
acted on by gravity. The path of the particle is thus a parabola, 
whose axis is parallel to the field and whose apex is at the point 
where the particle enters the electric field. The lear deviation 
1 ¢C. R. 130, p. 1129, 1900. 2 C. R..130, p. 809, 1900. 
