Precession Applied force Precession Port A Port A Exhaust air equal in all directions gyro erect Gyro precesses, increasing  exhaust from port A Precessing force at port A erects gyro, exhaust air again equal at all ports 10-58 Other characteristics of t he vacuum-driven gyro are shared by  the electric gyro. The  rotor is still oriented in the horizontal  plane. The free gyro gimbals allow the aircraft and instrument  case to rotate around the gyro rotor that remains rigid in space.  The gyroscopic direction indicator or directional gyro (DG)  is often the primary instrument for direction. Because a  magnetic compass fluctuates so much, a gyro aligned with  the magnetic compass gives a much more stable heading  indication. Gyroscopic direction indicators are located at the  center base of the instrument panel basic T.  Electric attitude indi cators are very similar to vacuum- driven gyro indicators. The main difference is in the drive  mechanism. Inside the gimbals of an electric gyro, a small  squirrel cage electric motor is the rotor. It is typically driven by  115-volt, 400-cycle AC. It turns at approximately 21,000 rpm. A miniature airplane fixed to the instrument case indicates the  aircraft’s attitude against the moving horizon bar behind it.  Electric attitude indicators address some of the shortcomings of  vacuum-driven attitude indicators. Since there is no air flowing  through an electric attitude indicator, air filters, regulators,  plumbing lines and vacuum pump(s) are not needed.  Contamination from dirt in the air is not an issue, resulting  in the potential for longer bearing life and less precession.  Erection mechanism ports are not employed, so pendulous  vanes responsive to centrifugal forces are eliminated.  It is still possible that the gyro may experience precession  and need to be erected. This is done with magnets rather than  vent ports. A magnet attached to the top of the gyro shaft  spins at approximately 21,000 rpm. Around this magnet,  but not attached to it, is a sleeve that is rotated by magnetic  attraction at approximately 44 to 48 rpm. Steel balls are free  to move around the sleeve. If the pull of gravity is not aligned  with the axis of the gyro, the balls fall to the low side. The  resulting precession re-aligns the axis of rotation vertically. Typically, electric attitude indicator gyros can be caged  manually by a lever and cam mechanism to provide rapid  erection. When the instrument is not getting sufficient power  for normal operation, an off flag appears in the upper right  hand face of the instrument.  [Figure 10-102] In addition to the contamination potential introduced by the  air-drive system, other shortcomings exist in the performance  of vacuum-driven attitude indicators. Some are induced by  the erection mechanism. The pendulous vanes that move  to direct airflow out of the gyro respond not only to forces  caused by a deviation from the intended plane of rotation,  but centrifugal force  experienced during turns also causes the  vanes to allow asymmet ric porting of the gyro vacuum air. The  result is inaccurate display of the aircraft’s attitude, especially  in skids and steep banked turns. Also, abrupt acceleration and  deceleration imposes forces on the gyro rotor. Suspended in  its gimbals, it acts similar to an accelerometer, resulting in a  false nose-up or  nose-down indication. Pilots must learn to  recognize these errors and adjust accordingly.  Early vacuum-driven attitude indicators were limited in how  far the aircraft could pitch or roll before the gyro gimbals  contacted stops, causing abrupt precession and tumbling of  the gyro. Many of these gyros include a caging device. It is  used to erect the rotor to its normal operating position prior  to flight or after tumbling. A flag indicates that the gyro must  be uncaged before use More modern gyroscopic instruments  are built so they do not tumble, regardless of the angular  movement of the aircraft about its axes. Figure 10-101.  The erecting mechanism of a vacuum-driven attitude indicator.  Gyroscopic Direction Indicator or Directional Gyro  (DG) Electric Attitude Indicators