Velocities oj a Particles from Radioactive Substances. 561 



independent determination was made in the laboratory by 

 Marsden and Taylor * in the course of their measurements 

 of the changes of velocity o£ the a particle in passing through 

 matter. The scintillation method was used, and the deflexion 

 of the a rays in a magnetic field was measured directly with 

 the aid of a microscope. The value obtained was 4*00 x 10% 

 with a probable error of J per cent. 



Electrostatic Deflexion of the Rays. 



In the previous experiments of Rutherford in 1906 the 

 maximum electrostatic deflexion of the trace of a rays, 

 obtained on a photographic plate by reversal of the electric 

 field, was 3 mm. The a rays were passed through the 

 exhausted space between two parallel plates about 0"2 mm. 

 apart and 3'8 cm. long, which served both for a slit and for 

 application of the electric field. The disadvantage of the 

 method lay in the fact that it was necessary to measure the 

 distance between the outside edges of the photographic 

 impression obtained by reversal of the field. In the light 

 of later knowledge, this measurement was subject to some 

 uncertainty on account of the marked scattering of the 

 a rays by the sides of the metal plates. 



In the present experiments it was our object to obtain an 

 electrostatic deflexion of the a rays on reversal of more than 

 1 cm., and under experimental conditions where all the 

 quantities involved could be measured with considerable 

 precision. With the steady voltage at our disposal (about 

 3000 volts) this result could only be obtained by observing 

 the deflexion of a pencil of a rays at a distance of nearly 

 1 metre from the source. At such a distance the total photo- 

 graphic effect due to a very active wire coated with radium C 

 is too small for detection. Another difficulty arises from 

 decay of the activity during the time required to exhaust the 

 large apparatus which has to be used to a pressure low 

 enough to prevent a discharge between the plates when the 

 high voltage is applied. In the preliminary experiments, to 

 avoid these difficulties, the source was placed outside the 

 exhausted vessel, the rays passing through a mica window of 

 known thickness, and the deflexion of the pencil of rays by 

 the electric field was determined by the scintillation method. 

 Mr. Marsden, who has great experience and skill in scin- 

 tillation work, very kindly assisted us in making a number 

 of the measurements. The zinc sulphide screen was at 

 a distance of about 1 metre from the source, and at this 



* Marsden and Taylor, Proc. Roy. Soc. A. hxxviii. p. 443 (1913). 

 Phil. Mag. S. 6. Vol. 28. No. 166. Oct. 1914. 2 



