400 Hz AC 400 Hz AC Amplifier Dial of remote indicator Fixed phase Variable phase Slaving torque  motor Autosyn indicator Autosyn transmitter Directional gyro Slaved gyro control Flux valve Stator Rotor 400 Hz AC Earth’s magnetic field Figure 10-84.  A simplified schematic of a flux gate, or slaved gyro, compass system.  Vacuum Systems Venturi Tube Systems 10-48 A light, single-engine aircraft can be equipped with a  2-inch venturi (2 inches of mercury vacuum capacity) to  operate the turn and bank indicator. It can also have a larger  8-inch venturi to power the attitude and heading indicators.  Simplified illustrations of these venturi vacuum systems  are shown in  Figure 10-87 . Normally, air going into the  instruments is filtered. The vacuum value required for instrument operation is usually  between 3½ inches to 4½ inches of mercury. It is usually  adjusted by a vacuum relief valve located in the supply line. Some turn-and-bank indicators require a lower vacuum  setting. This can be obtained through the use of an additional  regulating valve in the turn and bank vacuum supply line.  Vacuum systems are very common for driving gyro  instruments. In a vacuum system, a stream of air directed  against the rotor vanes turns the rotor at high speed. The  action is similar to a water wheel. Air at atmospheric pressure  is first drawn through a filter(s). It is then routed into the  instrument and directed at vanes on the gyro rotor. A suction  line leads from the instrument case to the vacuum source.  From there, the air is vented overboard. Either a venturi or a  vacuum pump can be used to provide the vacuum required  to spin the rotors of the gyro instruments. The velocity of the air rushing through a venturi can create  sufficient suction to spin instrument gyros. A line is run from  the gyro instruments to the throat of the venturi mounted on  the outside of the airframe. The low pressure in the venturi  tube pulls air through the instruments, spins the gyros, and  expels the air overboard through the venturi. This source of  gyro power is used on many simple, early aircraft.