After these settings are made, it should be possible to reproduce the proper meter indications at 0.01 and 10 mr/hr without any further adjustment. The instrument should then be calibrated over its full range by obtaining the recorder current for several values of radiation and plotting a curve which should match curve A shown in Figure 4. If there is a departure from linearity, compensate by readjusting the B plus set control, R-29, slightly. However, if this is done the switch should be then turned to the SET position and R-30 readjusted to froduoce a recorder current of 0.70 ma. Should there be a marked non- linearity in the calibration curve, it will be necessary to select new amplifier tubes, V-1 and V-2 until a pair is found which will gve the fequired characteristics. To calibrate the high level detector, set the range switch to position C and repeat the procedure for setting the photomultiplier, tube gain control and the float control described above; however, use radiation values of 10 mr/hr and 10 r/hr to correspond to currents of 0.200 and 0.740 ma respectively. Selection of the proper photomultiplier tube, VR tube (V-4) and gain control adjustment (R-19) should bring the detector into proper calibration as shown by curve B in Figure 4, without any further change of B plus or amplifier tubes. 20505 Buck-Out Circuit Set the arm of K-34] at 1,850 ohms between it and ground as measured by an ohmmeter. The buck-out potentiometers are then adjusted to produce zero volts measured between the arm of switch S-1H and ground - 23 -