e g incremental transmitter resolver or tachometer generator in front of the inlet delivery device 4 a unit 22 is arranged for maintaining constant a predetermined pre tensioning force in the running specimen 10 as illustrated in fig2 this means 22 may be adjusted pneumatically or electronically the means being controllable by the measuring signal from the force sensor 14 picked at the specimen 10 the means 22 is set with both delivery devices 4 8 running at the same speed i e without delay until the force measured by the force sensor 14 matches the pre tensioning force to be set the means 22 comprises an inlet side disk tensioner 40 via which the specimen 10 is supplied the specimen 10 is wound upon a reel at least two complete windings being on the reel 42 from the motor driven reel 42 the specimen is passed through an eyelet 43 in a feeler lever 44 and via an outlet side guiding eyelet 48 to a measuring means 26 or the inlet delivery device 4 respectively the feeler lever 44 is pre tensioned by means of a pressure spring 46 the spring force of the pressure spring 45 may be adjusted manually or electronically depending on a control signal so that a desired pre tensioning force may be set in the specimen 10 the position of the feeler lever 44 controls the motor drive of the reel 42 so that an exactly constant pre tensioning force acts on the specimen 10 as an alternative to the means 22 and prior to the beginning of a test the ratio of the speeds of the inlet and the.