PURPOSE: To eliminate the influence of the deviation in distributions of powder by providing two electrodes which are stuck tightly on the outside circumferential surface of a piping which is an insulator symmetrically with respect to the central axis of the piping and further constituting said electrodes of spiral or staircase-like sheets.

CONSTITUTION: Two electrodes 2', 2" are provided on the outside circumferential surface of a piping 1 which is an insulator symmetrically with respect to the central axis of the piping. These electrodes are constituted of thin sheets wound spirally or like a staircase on the outside circumference of the insulator 1. If such electrodes are used, the influence of the deviation in the distributions of powder in the pipe is eliminated.