the insertion of a strip of extra-fine 000 (triple aught) grit,  or finer, sandpaper under the brush, rough side towards the  carbon brush.  [Figure 9-53] Pull the sandpaper in the direction of armature rotation, being  careful to keep the ends of the sandpaper as close to the slip  ring or commutator surface as possible in order to avoid  rounding the edges of the brush. When pulling the sandpaper  back to the starting point, raise the brush so it does not ride  on the sandpaper. Sand the brush only in the direction of  rotation. Carbon dust resulting from brush sanding should  be thoroughly cleaned from all parts of the generators after  a sanding operation. After the generator has run for a short period, brushes should  be inspected to make sure that pieces of sand have not become  embedded in the brush. Under no circumstances should  emery cloth or similar abrasives be used for seating brushes  (or smoothing commutators), since they contain conductive  materials that cause arcing between brushes and commutator  bars. It is important that the brush spring pressure be correct.  Excessive pressure causes rapid wear of brushes. Too little  pressure, however, allows bouncing of the brushes, resulting  in burned and pitted surfaces. The pressure recommended by  the manufacturer should be checked by the use of a spring  scale graduated in ounces. Brush spring tension on some  generators can be adjusted. A spring scale is used to measure  the pressure that a brush exerts on the commutator.  Flexible low-resistance pigtails are provided on most heavy  current carrying brushes, and their connections should be  securely made and checked at frequent intervals. The pigtails  should never be permitted to alter or restrict the free motion of  the brush. The purpose of the pigtail is to conduct the current  from the armature, through the brushes, to the external circuit  of the generator.  Generator Controls Theory of Generator Control All aircraft are designed to operate within a specific voltage  range (for example 13.5–14.5 volts). And since aircraft  operate at a variety of engine speeds (remember, the engine  drives the generator) and with a variety of electrical demands,  all generators must be regulated by some control system. The  generator control system is designed to keep the generator  output within limits for all flight variables. Generator  control systems are often referred to as voltage regulators  or generator control units (GCU).  Aircraft generator output can easily be adjusted through  control of the generator’s magnetic field strength. Remember,  the strength of the magnetic field has a direct effect on  generator output. More field current means more generator  output and vice versa.  Figure 9-54  shows a simple generator  control used to adjust field current. When field current is  controlled, generator output is controlled. Keep in mind, this  system is manually adjusted and would not be suitable for  aircraft. Aircraft systems must be automatic and are therefore  a bit more complex. There are two basic types of generator controls:  electro-mechanical and solid-state (transistorized). The  electromechanical type controls are found on older aircraft  and tend to require regular inspection and maintenance.  Solid-state systems are more modern and typically  considered to have better reliability and more accurate  generator output control.  Unseated brush 000 sandpaper (sand side next to brush) Properly seated brush 1 /32" to  1 /16" Generator control Load Shunt field + − B Figure 9-53.  Seating brushes with sandpaper.  Figure 9-54.  Regulation of generator voltage by field rheostat. 9-34