VI. 2] QUANTIZATION IN SPACE 87 
behind the magnetic pole. When this layer was developed 
photographically it was found that there were two distinct, 
sharply-defined bands, indicating that the silver atoms were 
sorted out into two distinct and separate beams. The positions 
of the images on the plate showed that the atoms in one beam 
had been attracted towards the pole, and those in the other 
beam had been repelled, the attraction being slightly greater 
than the repulsion. When the electromagnet was not excited, 
no undeflected beam could be detected. 
These results, then, give remarkable support to the quantum 
theory and to the principle of quantization in space. It was 
concluded from these experiments that all the silver atoms in 
the stream possessed a definite moment. From their measure 
ments of the separation between the two limbs of the loop formed 
on the plate, Gerlach and Stern deduced the magnetic moment 
of the normal silver atom in the gaseous state, and found it to 
be, within the limits of experimental error, one Bohr magneton. 
This magneton is the value deduced on Bohr’s theory for an 
electron describing a 1 x circular orbit. In the earlier experiments 
the error was of the order of 10 per cent. ; but later investigation 
reduced the error to something like 2 per cent. 
In these experiments, the atoms, after passing through various 
openings, enter a chamber evacuated so that the free path of 
the molecules in it is very large compared with the dimensions 
of the space. The atoms pass through the opening into the 
chamber at such intervals that, in spite of the fact that their 
velocities are distributed according to Maxwell’s law, there are 
few or no collisions. For these atoms in the stream, which may 
be made quite narrow, Gerlach* suggests the name “atomic 
rays ” (Atomstrahlen). 
It would appear, then, that while atoms of silver are passing 
through the magnetic field their magnetic axes have two distinct 
orientations in space. The difficult question as to how the mag 
netic atoms can alter their direction at all under the influence of 
a magnetic field has been discussed by Einstein and Ehren- 
fest.f Experiments in continuation of the original work of 
Gerlach and Stern have been carried out by other investigators, 
notably by Gerlach and Cilliers,^ and by Gerlach § himself. 
The results of these investigations are of such great importance 
that some further account of the method appears desirable. 
In Fig. 14 (p. 88) we have a diagrammatic representation of the 
apparatus. 
* Gerlach, Ann. d. Physik, vol. 76, p. 106, 1925. 
f Einstein and Ehrenfest, Zeits. f. Physik, vol. II, p. 31, 1922. 
+ Gerlach and Cilliers, Zeits. f. Physik, vol. 26, p. 106, 1924. 
§ Gerlach, Ann. d. Physik, vol. 76, p. 163, 1925.