=90. Electrostatic deviation of the α rays=. If the rays are charged bodies, they should be deflected in passing through a strong electric field. This was found by the writer to be the case, but the electric deviation is still more difficult to detect than the magnetic deviation, as the intensity of the electric field must of necessity be less than that required to produce a spark in the presence of radium. The apparatus was similar to that employed for the magnetic deviation (Fig. 32) with this exception, that the brass sides which held the plates in position, were replaced by ebonite. 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 _m_ _H_ρ = ---- _V_ . _e_ 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, 2ρ_d₁_ = _l₁²_ or _l₁²_ _e_ _H_ _d₁_ = ----- ---- ---- (1). 2 _m_ _V_ If the rays pass through a uniform electric field of strength _X_ and length _l₂_ with a deviation _d₂_, 1 _Xel₂²_ _d₂_ = --- ------ (2), 2 _mV²_ 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) _d₁_ _l₂²_ _X_ _V_ = ---- ----- -----, _d₂_ _l₁²_ _H_ and _e_ 2_d₁_ _V_ ---- = ---- --- . _m_ _l₁²_ _H_ 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