PURPOSE: To reduce and eliminate magnetism by a method wherein hot wires are divided into plural groups, and a set is provided by making the adjoining group of the hot wires into a unit of the set so that electric currents electrifying these groups adjoining to each other within the set become opposite to each other.

CONSTITUTION: Hot wires 3 are divided into groups of hot wires 3a, 3b, 3c and 3d, and sets 3A and 3B making the groups of hot wires 3a, 3b and 3c, 3d into their units respectively are provided. Then on one side of each of the groups of the hot wires 3aW3d the groups of the hot wires 3a, 3b and 3c, 3d connected with a plus power source side bus bar 4a1, a minus power source side bus bar 4b1 and a relay bus bar 5a respectively, and on the other side of them the groups of the hot wires 3a and 3b, 3c and 3d are connected with a relay bus bar 5b, a bus bar 4a2 and a bus bar 4b2 respectively. Consequently electric currents electrifying the groups of the hot wires 3a, 3b, 3c and 3d become opposite to each other.