A laboratory tube has been developed which produces scale models of the forms assumed by streams of charged particles in the earth's dipole magnetic field. The forms are unexpectedly intricate and complicated, as are also the contacts on the earth's surface. Photographs of some representative stream forms and contacts are presented. The tube has served as an analog computer for stream forms and contacts and, in addition, has revealed the importance of the generation of ring currents of charged particles scattered into Sto¨rmer's periodic orbits. The same techniques can be applied in studies of stream forms in high‐energy accelerators and in other complicated kinds of magnetic fields.