A yarn sluice for sealing a pressurized yarn treating chamber in which a traveling yarn is thermally treated, comprises yarn guide elements in the region of a yarn inlet and outlet openings of the treating chamber forming a yarn guide channel sealed by the traveling yarn during operation. The battery includes an erasable programmable read-only memory and a signal terminal connected to the EPROM. Thus, a light signal which is amplitude modulated around zero is generated, the intensity of said light signal being proportional to the voltage induced in the MR receiving coil. Thus, both increase in the resistance change amount and magnetic field stability can be achieved. The roller member exhibits enhanced durability, as well as improved cleaning characteristics and charge-imparting or charge-eliminating characteristics.