The voltage disturbances in hot‐cathode arcs are markedly affected by a magnetic field transverse to the normal flow of current. The normal plasma oscillations can be suppressed and the noise level above 1 Mc/sec. greatly increased. The oscillations appear to be transmitted to the electrodes by electrons whose velocity is modulated by the plasma ion oscillations as they travel directly from cathode to anode. The effect of the magnetic field is to remove these electrons from the current reaching the anode. A critical value of magnetic field will suppress the oscillations and reduce the noise to a minimum. As the magnetic field is further increased the high frequency noise level increases. The high frequency noise level also increases if the pressure decreases, or if a lighter gas is used. The noise level is independent of the plasma volume. The effect of the magnetic field is to build up a region of high ion density in a narrow region near the electron source. Plasma oscillations with continuous spectrum are generated in this region of high density and high density gradient.