## Extract

1. The first accurate determinations of the velocity with which an α particle is expelled from a radio-active substance and of the value of E/M, the ratio of the charge to the mass, were made by Rutherford and Robinson by measuring the deflections in magnetic and electric fields. An α particle moving with velocity V perpendicular to the direction of a magnetic field H describes a circle of radius *ρ* where H*ρ* = MV/E. Rutherford and Robinson found for α particles expelled from radium C Hρ = 3·983 X 10^{5} E. M. U. and V = 1·922 X 10^{9} cm. per second. The mean value of E/M for α particles from radium emanation, radium A and radium C was 4820 E. M. U., which agreed to within the limits of experimental error with the value 4826 deduced from electrochemical data taking the atomic weight of helium as 3·998 and the value of the faraday as 9647. This value of the velocity of α particles from radium C has served as a standard from which the velocities of α particles from other radio-active substances have been calculated from the Geiger relation V^{3} = *k*R. The present paper gives an account of a redetermination of the quantity Hρ for α particles from radium C by a method which is essentially similar to those used in previous determinations of this kind. From the value of Hρ the velocity has been calculated using the theoretical value of E/M which can be found to a high degree of accuracy from more recent determinations of the atomic weight of helium and the value of the faraday, taking into account the relativity correction for the increase in mass of the α particle. For the atomic weight of helium we may take 4·000, the value deduced by Van Laar from a consideration of the density determinations of Watson, Heuse, Taylor, and in particular of Guye’s discussion of Taylor’s results. Taking the value of the faraday given by recent determinations 9649·4 E. M. U. the value of E/M for a slow moving α particle is 4824·7 and on applying the correction for the relativity change of mass this becomes 4814·8 for the α particle from radium C, neglecting the mass of the lost electrons. It may be noted that Rutherford and Robinson’s experimental result, 4820 agrees as well with this latter value as with the value 4826 which they calculated.

## Footnotes

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- Received February 2, 1928.

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