Measuring instruments

Claim 1

IN AN INSTRUMENT OF THE TYPE DESCRIBED HAVING A SOURCE OF LIGHT AND MEANS FOR MOVING A FIBER SAMPLE ACROSS THE PATH OF LIGHT FROM SAID LIGHT SOURCE; THE IMPROVEMENT WHICH COMPRISES AN ELECTRIC CIRCUIT HAVING LIGHT RESPONSIVE MEANS IN POSITION TO RECEIVE LIGHT PASSING THROUGH THE SAMPLE AND HAVING ADJUSTABLE MEANS FOR BRINGING THE ELECTRICAL POTENTIAL OF A POINT IN THE CIRCUIT TO A PREDETERMINED VALUE; SAID CIRCUIT INCLUDING A FIRST POTENTIOMETER COIL CONNECTED AT ONE END TO A SOURCE OF ELECTRICAL POTENTIAL, A FIRST TAP ON SAID COIL, A SECOND POTENTIOMETER COIL CONNECTED AT ONE END TO SAID FIRST TAP, AN ADJUSTABLE CALIBRATING RESISTOR CONNECTED TO THE OPPOSITE ENDS OF SAID COILS AND TO GROUND, A SECOND TAP ON SAID SECOND COIL, AN ADJUSTABLE CALIBRATING LOAD RESISTOR CONNECTED TO SAID SECOND TAP AND TO GROUND, A THIRD POTENTIOMETER COIL CONNECTED IN PARALLEL WITH SAID LOAD RESISTOR, A THIRD TAP ON SAID THIRD COIL, A RESISTOR CONNECTED BETWEEN SAID THIRD TAP AND SAID LIGHT RESPONSIVE MEANS, A FOURTH CALIBRATING POTENTIOMETER COIL CONNECTED TO A SOURCE OF ELECTRICAL POTENTIAL, A FOURTH TAP ON SAID FOURTH COIL, AND A RESISTOR CONNECTED BETWEEN SAID FOURTH TAP AND SAID LIGHT RESPONSIVE MEANS; SAID FIRST AND SECOND TAPS ON SAID FIRST AND SECOND POTENTIOMETER COILS BEING MECHANICALLY COUPLED TOGETHER FOR MOVEMENT IN UNISON; AND MEANS FOR INDICATING THE POSITION OF SAID FIRST AND SECOND TAPS.