CONTEMPORARY ADVANCES IN PHYSICS 



327 



Continuing to look at the upper part of Fig. 1 , we have two large mag- 

 net-poles, and the beam travels between them, having no trouble with 

 molecules of air as it shoots along, for the whole of this apparatus is en- 

 closed in a highly-evacuated tube. It may seem natural to visualize 

 these magnet-poles as the two broad flat extremities of a horseshoe-mag- 

 net, with a uniform magnetic field pervading all the space between them. 

 Such an arrangement, however, would make the experiment futile. 





NORTH POLE 



In 



S N 



--^^~--- 



Fig. 1 — Longitudinal section and cross-section of apparatus for the 

 Gerlach-Stern experiment. 



Nothing would happen to any of the atoms, for in the uniform field 

 the north pole of each atomic magnet would be pressed downward 

 just as hard as and no harder than the south pole is drawn upward, 

 and the net force would be zero. The beam would go on unbroadened 

 and undeflected, and make a small spot on the screen S, the spot 

 being just opposite the slits in the diaphragms. Something else must 

 be tried; and what we do — or rather, what Gerlach and Stern did in 

 Hamburg some fourteen years ago — is, to shape one of the magnet- 

 poles in the form of a wedge and hollow out the other, so that the field 

 between the two shall no longer be uniform. The lower part of Fig. 1 

 represents the cross-section of such a pair of pole-pieces. The field- 

 strength is now much greater near the wedge than near the opposite 



