An apparatus for conducting a fusion process on a fiber includes a fiber chamber having coupled thereto a first fiber holder having a groove through which a fiber can be inserted into the fiber chamber and suspended therein, a filament chamber which maintains an inert and/or reducing atmosphere, a resistive filament movably supported in the filament chamber, a partition adjoining the fiber chamber and the filament chamber, the partition being provided with an orifice, a valve which selectively opens and closes the orifice, and a positioning device which moves the resistive filament between the filament chamber and the fiber chamber when the valve opens the orifice.