A B C Flux to right No flux Flux to left N S S N N S S N S N N S 0° 45° 90° Coil core Pole shoe Flux density 45° 90° 360° 315° Full register 0° 135° 225° Neutral Figure 4-3.  Magnetic flux at three positions of the rotating magnet.  Figure 4-4.  Change in flux density as magnet rotates. flux passing through the coil core begins to decrease. This  occurs because the magnet’s poles are moving away from  the pole shoes, allowing some lines of flux to take a shorter  path through the ends of the pole shoes. As the magnet moves farther from the full register position,  more lines of flux are short circuited through the pole shoe  ends. Finally, at the neutral position 45° from the full register  position, all flux lines are short circuited, and no flux flows  through the coil core.  [Figure 4-3B]  As the magnet moves  from full register to the neutral position, the number of flux  lines through the coil core decreases in the same manner  as the gradual collapse of flux in the magnetic field of an  ordinary electromagnet. The neutral position of the magnet is where one of the poles  of the magnet is centered between the pole shoes of the  magnetic circuit. As the magnet is moved clockwise from  this position, the lines of flux that had been short circuited  through the pole shoe ends begin to flow through the coil  core again. But this time, the flux lines flow through the coil  core in the opposite direction.  [Figure 4-3C]  The flux flow  reverses as the magnet moves out of the neutral position  because the north pole of the rotating permanent magnet is  opposite the right pole shoe instead of the left.  [Figure 4-3A] Figure 4-4  shows that as the magnet moves from the full  register position 0°, flux flow decreases and reaches a zero  value as it moves into the neutral position 45°. While the  magnet moves through the neutral position, flux flow reverses  and begins to increase as indicated by the curve below the  horizontal line. At the 90° position, another position of  maximum flux is reached. Thus, for one revolution 360° of  the four pole magnet, there are four positions of maximum  flux, four positions of zero flux, and four flux reversals. This discussion of the magnetic circuit demonstrates how the  coil core is affected by the rotating magnet. It is subjected to  an increasing and decreasing magnetic field and a change in  polarity each 90° of magnet travel. When the magnet is again moved a total of 90°, another full  register position is reached with a maximum flux flow in  the opposite direction. The 90° of magnet travel is shown in  Figure 4-4,  where a curve shows how the flux density in the  coil core, without a primary coil around the core, changes  as the magnet is rotated. When a coil of wire as part of the magneto’s primary  electrical circuit is wound around the coil core, it is also  affected by the varying magnetic field. 4-3