power source 80 as adjusted by variable resistor 78 on lines 82 to a milli volt meter 84 utilized as an amplifier the thus amplified signal is carried by output lines 85 to a strip chart recorder 86 and illustrated diagrammatically at 88 is a strip chart illustrating a typical trace 89 of the recorded signal fig5 illustrates still a further modification of the device for carrying out the method of this invention wherein an optical fiber 90 is positioned in a rotating spindle 92 whose inner end is terminated by a filter 94 positioned adjacent a photovoltaic detector 96 the signal thus produced which is a direct analog of the light transmitted by optical fiber 90 is applied to the input of an operational amplifier 100 illustrated diagrammatically and the output of operational amplifier 100 is utilized to the input of a log voltmeter 104 in turn the output of log voltmeter 104 is put into strip chart recorder 108 to produce strip 110 in addition the strip chart recorder 108 may have its speed controlled by utilizing a d c signal from the fiber pull speed control 112 which is processed by a voltage controlled oscillator 114 to provide a signal on leads 116 for the input 118 to the external chart speed input control illustrated on chart 110 is a signal trace 120 representing a finite length of the optical fiber loss and at 122 a short length loss trace while in the various forms of apparatus illustrated in the figures the outputs are represented by observer meters or in.