PHYSICS: MILLIKAN, GOTTSCHALK AND KELLY 
591 
physical interest, it does have a purely mathematical interest, and I 
shall therefore determine also these orbits. 
The expression which must be integrated is * 
^.^ = (vX^).(vx5Y (8) 
dt dt \ dtj \ dt) 
provided the units are so chosen that the velocity of light is 1 . Let R 
be the radius of curvature of the orbit (in ordinary space). The 
tangential and normal resolution of acceleration throws (8) into 
\dt) R^ \ds) R^ ' 
Hence 
If then any space curve be given intrinsically by the equation R = f(s)y 
equation (9) determines the velocity at which the curve must be traced 
if there is to be no radiation as estimated by the usual electro-magnetic 
formula. 
1 See H. A. Lorentz, Theory of Electrons, Art. 37, B. G. Teubner, Leipzig, 1916. 
2 See M. Planck, Theorie der Wdrmestrahlung, p. 110, J. A. Barth, Leipzig, 1906. 
3 Wilson, E. B., Boston, Proc. Amer. Acad. Arts Sci., 50, 1914, (105-128). 
4 See Wilson and Lewis, Ibid., 48, 1912, (387-507), especially p. 481. 
THE EFFECT UPON THE ATOM OF THE PASSAGE OF AN 
ALPHA RAY THROUGH IT"- 
By R. a. Millikan, V. H. Gottschalk and M. J. Kelly 
Ryerson Physical Laboratory, University of Chicago 
Read before the Academy, November 11, 1919 
In 1910, by catching at the instant of ionization the positive residues 
of atoms ionized by X-rays, and by beta and gamma rays of radium, it 
was conclusively shown^ that the act of ionization by these agencies 
uniformly consists in the detachment of a single negative electron from 
a neutral atom. 
The method consisted in balancing the force of gravity acting upon a 
minute oil-drop by a strong vertical electrical field, holding the oil-drop 
under observation in a telescope with the aid of a powerful beam of light, 
