A meter is included on the cover of the control box to indicate ground level radiation intensity. The amplifier bias, acale buck-out and B plus voltmeter controls are mounted on a board within the control box, and adjusted only during calibration. Batteries are accessible by releasing the snap locks on each end of the potentiometer board. 2.5 Calibration The calibration procedure is divided into four phases: 1) The preliminary power supply adjustments, 2) The "Top Hat" detector, 3) The buck-out ciruit setting and 4) the altitude compensation circuit. Calibration of this instrument requires a standard gamma radiation source such as radium or cobalt 60 of suitable size, a vacuum tube voltmeter and an ohmmeter. The locations of the internal controls are shown in Figure 10. 2.3.1 Preliminary Power Supply Adjustments Connect the required cables between the "Top Hat", Control Box and the Aircraft Radiation Recorder. High Voltage Supply - If the vibrator does not start, ‘rotate the vitran contact two turns counter-clockwise. This control is reached through the hole in the high voltage battery board (See Figure 10). Set switch 8-1 to position SBT, turn the vitran contact clockwise until the reed is heard to just start to vibrate and then advance the contact 1/8 turn. When the switch is now turned from OFF to SET, the vitran should start without further adjustment, B plus Supply - With the switch at SET position, adjust R-29 to produce the minimum B plus voltage. This point will be indicated by the minimum recorder deflection that can be obtained with this control. - 20 -