An electron-hole-plasma can be produced in cylindrical specimens of n-type germanium by carrier injection through a p- n junction. It is possible with suitable potential probes to excite a helical density wave in this plasma by an a.c. voltage externally, if there exists a longitudinal magnetic field. The properties of the surface density wave, especially its variation on time- and space-coordinates are determined with further potential probes in case of low exciting amplitudes. At high injection level the surface density wave becomes unstable in the vicinity of the p- n junction because the spatial amplification there is very large. This phenomenon can be connected with the onset of the oscillistor-effect. However, at low injection level a surface density wave can be observed, which is amplified out of the noise without exciting from outside and before an absolute instability sets in. The consequence for the electrical resistance of the sample is discussed. There is a very good agreement for the excited and also for the spontaneous oscillations between experiments and theory in the validity range of the approximations used here.