of 4 mm to 10 mm are slowly inserted into the crucibles the glass rod 40 is inserted into the inner crucible through the tubular shaft 8 the crucible nozzles can be closed off by means of a seal not illustrated as soon as the glass melts have been brought to the drawing temperature this seal is removed so that a light conducting fiber 11 can be drawn a drawing drum 15 is provided for this purpose a thickness measuring device 12 can be provided for measuring the thickness of the light conducting fibers if it is desired to provide the light conducting fibers with an additional protective casing a coating bath 13 can be provided polyvinyliden chloride trade name kynar is a suitable material for example for a protective casing a drying furnace 14 can be used to achieve a more rapid hardening of the protective casing the crucibles 1 and 2 must consist of a material which does not pollute the glass melts in the crucible suitable materials for the crucibles are for example platinum and iridium which metals can also be alloyed with rhodium first investigations have indicated that highly pure aluminum oxide is also suitable as a crucible material as yet no corrosion phenomena have been able to.