PROCEEDINGS 
OF THE 
NATIONAL ACADEMY OF SCIENCES 
Volume 7 FEBRUARY 15, 1921 Number 2 
POSITIVE RAY ANALYSIS OF MAGNESIUM 
By Arthur J. Dempster 
Rybrson Physical Laboratory, University of Chicago 
Communicated by A. A. Michelson, Dec. 7, 1920 
In the Physical Review for April, 1918, I described an apparatus for 
positive ray analysis and gave examples of hydrogen, oxygen, sodium and 
potassium rays, showing that at least the lighter elements could be readily 
analyzed so as to separate any molecules differing in molecular weight by 
unity. I have recently resumed the experiments and will give in this 
paper an account of experiments with magnesium. 
As stated in the Physical Review article, the experimental difficulties 
are largely in obtaining a steady source of the rays desired. Magnesium 
rays have been obtained from a piece of the metal which was heated 
electrically by a coil of wire, and at the same time bombarded by electrons 
from a Wehnelt cathode. The occluded gases are first driven off, and 
then the heating current is increased till the magnesium lines appear due 
to the metal vaporizing slightly. The positively charged molecules 
formed pass through a hole in a plate below the cathode and are then 
accelerated by a strong variable field of several hundred volts. The 
plate has been added to the apparatus described in the Physical Review 
to prevent the strong electrical field influencing or even inhibiting the low 
voltage discharge. The first slit, about 1 mm. wide, separates out a 
bundle of rays which is bent into a semicircle by a strong magnetic field 
and refocussed, if their speed is right, on the second slit, below the detect- 
ing electrode. The charge carried by the rays, which is proportional 
to the number of the molecules of different kinds, is measured by a Wilson 
electroscope used as a null instrument with a special compensating device 
for rapid measurements. 
The charged atoms of different atomic weights are successively brought 
on to the detecting electrode by keeping the magnetic field constant and 
varying the potential which accelerates the rays, the potential required 
being inversely proportional to the mass of the particles. Thus, if one 
atomic weight is known the others may be found. Due to the finite width 
of the slits, each element gives a curve, on the atomic weight scale, which 
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