Alternate plates were connected together and charged to a high potential by means of a battery of small accumulators. The discharge in the electroscope, due to the α rays, was found to be diminished by application of the electric field. With plates ·055 cm. apart and 4·5 cms. high, the diminution was only 7% with a P.D. of 600 volts between the slits. With a special arrangement of plates, with slits only ·01 cm. apart, the discharge was diminished about 45% with an electric field corresponding to 10,000 volts per cm.
91. Determination of the constants of the rays. If the deviation of the rays in both an electric and magnetic field is known, the values of the velocity of the rays, and the ratio e/m of the charge of the particle to its mass can be determined by the method, first used by J. J. Thomson for the cathode rays, which is described in section 50. From the equations of a moving charged body, the radius of curvature ρ of the path of the rays in a magnetic field of strength H perpendicular to the path of the rays is given by
If the particle, after passing through a uniform magnetic field for a distance l1, is deviated through a small distance d1 from its original direction,
If the rays pass through a uniform electric field of strength X and length l2 with a deviation d2,
since Xe/m is the acceleration of the particle, at right angles to its direction, and l2/V is the time required to travel through the electric field.
From equations (1) and (2)
The values of V and e/m are thus completely determined from the combined results of the electric and magnetic deviation. It was found that
On account of the difficulty of obtaining a large electrostatic deviation, these values are only approximate in character.
The results on the magnetic and electric deviation of the α rays of radium have been confirmed by Des Coudres34, by the photographic method. Some pure radium bromide was used as a source of radiation. The whole apparatus was enclosed in a vessel which was exhausted to a low vacuum. In this way, not only was he able to determine the photographic action of the rays at a much greater distance from the source, but he was also able to apply a stronger electric field without the passage of a spark.