Figure 8-31.  Angle of attack indicators.  state of an airplane is the balance between airspeed, altitude,  drag, and thrust and represents how efficiently the airfoil is  operating.  There are long and short graduation marks between the letters  and numbers, each long mark representing 10° and each short  mark representing 5°.  The Earth is a huge magnet, spinning in space, surrounded  by a magnetic field made up of invisible lines of flux. These  lines leave the surface at the magnetic North Pole and reenter  at the magnetic South Pole.  Lines of magnetic flux have two important characteristics:  any magnet that is free to rotate will align with them, and  an electrical current is induced into any conductor that cuts  across them. Most direction indicators installed in aircraft  make use of one of these two characteristics.  One of the oldest and simplest instruments for indicating  direction is the magnetic compass. It is also one of the basic  instruments required by Title 14 of the Code of Federal  Regulations (14 CFR) part 91 for both VFR and IFR flight.  A magnet is a piece of material, usually a metal containing  iron, that attracts and holds lines of magnetic flux. Regardless  of size, every magnet has two poles: north and south. When  one magnet is placed in the field of another, the unlike poles  attract each other, and like poles repel.  An aircraft magnetic compass, such as the one in  Figure 8-32,  has two small magnets attached to a metal float sealed inside a  bowl of clear compass fluid similar to kerosene. A graduated  scale, called a card, is wrapped around the float and viewed  through a glass window with a lubber line across it. The card  is marked with letters representing the cardinal directions,  north, east, south, and west, and a number for each 30°  between these letters. The final “0” is omitted from these  directions. For example, 3 = 30°, 6 = 60°, and 33 = 330°.  The magnets align with the Earth’s magnetic field and the  pilot reads the direction on the scale opposite the lubber  line. Note that in  Figure 8-32,  the pilot views the compass  The compass housing is entirely full of compass fluid. To  prevent damage or leakage when the fluid expands and  contracts with temperature changes, the rear of the compass  case is sealed with a flexible diaphragm, or with a metal  bellows in some compasses.  The float and card assembly has a hardened steel pivot in its  center that rides inside a special, spring-loaded, hard glass  jewel cup. The buoyancy of the float takes most of the weight  off of the pivot, and the fluid damps the oscillation of the  float and card. This jewel-and-pivot type mounting allows the  float freedom to rotate and tilt up to approximately 18° angle  of bank. At steeper bank angles, the compass indications are  erratic and unpredictable.  Figure 8-32.  A magnetic compass. The vertical line is called the  lubber line.  8-23  Magnetic Compass  Compass Systems  N-S  E-W 