them from being spewed into the atmosphere it is here noted that irregularities caused by deposits such as carbonized particles on the surface of the insulator 3 serve to limit the generation of the bridged stream coronas c1 resulting in a reduction of the function of purifying exhaust gas however such irregularities are smoothed out by the viscous oil film 5a defined by the viscous oil 5 added dropwise from the upper end of the insulator 3 so that the generation of the bridged stream coronas c1 can be maintained the viscous oil 5 also serves to increase the dielectric constant between the first and second electrodes 1 and 2 thereby improving the efficiency of generating the bridged stream coronas c1 experimentation made by the present inventor has indicated that when a 50 hz alternating current boosted to about 10 to 15 kv by the high voltage power source 4 shown in fig1 is applied between the first and second electrodes 1 and 2 far reaching massive bridged stream corona discharge takes place it has also been found that the thus obtained bridged stream coronas c1 suffer no attenuation and are stably maintained and ensured by the effect of the viscous oil film.